If that solenoid is OK, do the same with the front one. If they both test OK, is the system actually trying to change over? Common problem is the wire breaks off the temperature sensor on the reducer so the system thinks it isn't hot enough to change over. If it attempts to change over then immediately changes back, that is likely to be a stuck solenoid (no gas pressure being detected), if it doesn't attempt to change over then I'd be looking at the temperature sensor.

Fuse 41, 20A, supplies power to the contacts of the relay.

The 'relay' is the solenoid coil that operates the shut off valve on the tank. Check the resistance it should be around 11 Ohms and look for any cracks in the plastic body. If you put 12V onto it, you should hear the solenoid make a very audible clack as it opens. If you remove the small bolt in the centre of it, the coil just lifts off laving the valve body behind so no gas will leak out, but you'll be able to test and inspect it much better. Just behind it is a knurled knob which is a manual shut off valve. If you screw that in fully clockwise, that will stop the contents of the tank leaking out when you do the next step. At the base of the brass spindle that the coil sits on is a hex, usually 17 or 19mm. With a deep socket, slacken that off, some gas will escape (the gas in the pipe between the tank and the front shut off solenoid) so let it bleed out slowly. Once it stops, unscrew the spindle and take it out. There's a spring loaded plunger inside it that can get gummed up, clean it and put it back together.

On a pre-2000 model and SRS fault will still be there until it is cleared with diagnostics even after the actual fault is cleared. It's only on the later ones where the light goes out after you've fixed the fault without it needing to be cleared.

Horn button applies a ground via the rotary coupler. One side of it will have a ground on it while the other side should go to the relay so the ground causes it to operate.

I thought it would only go in one way. Have a close look at it and see if it looks like it has been butchered.

From the picture in RAVE, it looks like it is angled upwards?

Good call on only one fan running. That will also give the book and naff all air as what is sucked by one working fan will just go straight out the other side rather than through the heater box. If you take both pollen filters out, you can see them down the hole and see if they are both running.

You've put extended shocks on with standard air springs? Don't jack it up on the chassis or try any offroading, you will pull the air springs apart. It's the shocks that limit the amount of axle articulation you have.

Gorilla Glue is good for some things but it sets brittle so can break with movement and also expands as it sets.

Noit just the blend motors, the HEVAC will not try to use anything that it detects a fault on so if it detects a fault at self test when the ignition is first turned on, if a blend motor is detected as faulty it won't try to operate it. Same goes for the blowers, if it detects a fault with either blower, it doesn't try to use it. Ideally you need diagnostics to confirm what the fault is and which blend motor, only the faulty one will not be operating. You should be able to adjust the temperature on one side or the other. Unplugging the blend motors from the HEVAC, switching the ignition on for 10 seconds, then off and plug them back in sometimes resets the fault but not always.

The ducting is plastic with foam seals which rot away so most of the blower output can get out before it even gets as far as the heater box, using duct tape for what it was originally designed for can be used successfully. Blend motors can be changed by going in through the hole after you've removed the instrument cluster or by dropping the glovebox out, no need to take the whole dash out. First you need to identify which blend motor is at fault and what the fault is.

There never was sealer, the rubber seal was stuck to the bodywork with a strip of double sided tape. Unfortunately, with age the rubber shrinks slightly and pulls to tape away. I've stuck it back on both my cars with Evo Stik contact adhesive.

Yup, you've got it.

Just edited it so the pictures show up. Not sure what you are doing wrong but it's the same as JMCL was doing originally but recently he's been getting it right.

For pollen filters you just remove the pollen filter covers, the two long thin ones at the base of the plenum held in with a couple of self tapping screws, no need to remove anything else. They are a service item after all.

To post pictures you need to upload then to a hosting site like imgur.com (which is the one most of us seem to use, simply open a free account) then, when you are writing a post and want to insert an image, open imgur, click on the picture, click the button on the right marked Copy under Direct Link which will highlight the link, right click and copy that highlighted link, come back here and in your post click the insert picture button and a small box pops up, right click the URL box in there and select paste, clock OK and carry on writing your post. If you want to check it, click on Preview which will show you what your post will look like and, if you are happy with it, click Reply.

Microcat shows the black plastic bush pushing inside the copper mesh thing, that, and Rimmers describing it as an adapter, made me think maybe the later sensors were thinner and it allowed you to fit one of them. I thought the copper bit pushed straight into the hub but I must admit I've never taken that much notice of them, pulled them out, cleaned them up and put them back.

Absolutely no idea what FTC2249 is, never seen one of those and Rimmer Bros site describes it as an ABS sensor adapter. Adapter for what I've no idea. The sensor fits into LR029522 and should be lubricated with Silicone grease and pushed fully home.

Don't fit poly bushes to a P38, they are either crap or wear out in no time (or both). OE rubber are cheaper and better but you need a press to fit them. If you are taking wheels off, axle stands under the axle and it doesn't really matter what height the air suspension is at. I changed the exhaust, including dropping the crossmember to change the Y section, with it on the ground on high.

As to your faults,
Shocks, Boge were original but Boge rears are no longer available so Boge front and, dare I say it, Britpart rear (they are cheap, easy to replace and have a 2 year warranty if they don't last).
Handling may be down to the shocks but more likely ball joints. These are the top and bottom ones that the hub swivels on, cheap to get replacements but you ideally need the proper press tool for pushing the old ones out and new ones in. Often missed on the MoT as you need to get the wheel off the ground then try to lift it with a crowbar under the wheel.
Sunroof is easy enough to drop the motor off by dropping the interior light. If you can hear the motor turn but nothing happens then likely the cable. Not an easy repair, better to get a complete unit from a breaker.
Brake pipes do rust, the two that run from front to back and a couple of little short ones on either side at the rear (assuming post 97), easy enough to make up new ones in Kunifer although you'll need a crows foot spanner to get to the upper unions on the little rear ones.
Book on the HEVAC needs diagnostics to see what the problem is but even if it is blend motors, no need to take the dash out, you can get at them with the instrument cluster out and the glovebox dropped.
Again, diagnostics needed to see why the SRS light is on. If post 2000 could well be a connector under one of the seats, just unplug every Yellow connector you find and give it a squirt of contact cleaner, 2000 and on had side airbags so have connector under the seats, earlier cars didn't but there is a connector under the dash on each side that will benefit from contact cleaner. On a later car the SRS light will go out once the fault is cleared, earlier ones need diagnostics to reset it.
Rear arches and front lip of the bonnet are about the only places you will notice corrosion on a P38 as they are both steel but not not structural and pretty normal on many.
If it is a later car with the DSP amp, it may well have died (as they do) and been swapped for one from a Discovery 2 which works but only on the front doors.
No gas in the air con system could well be the cause of the book symbol on the HEVAC as it will detect that the compressor clutch isn't engaging when told to as the circuit goes via a pressure switch. If you poke the AC Off button on the HEVAC it won't try to engage the clutch so you won't get the book showing.

You haven't got anything there that isn't normal for a P38 that hasn't been looked after by someone that understands them and they are the ones that don't get sold. Whenever a car is being sold there is usually a reason for it and in the case of a P38 it's because the seller has been told it's going to cost him lots of money because he doesn't do it himself and has to pay someone else to do the work.

The pressure switch is also used on some Saabs, no idea if the pump is the same though. The one on a Classic is very similar except for the mounting points so probably the same innerds. I pulled mine apart after it burnt out and it doesn't look the sort of thing that can be repaired easily..

Arnotts do that, Dunlops don't. In 10 years and almost 200k miles, my car has never dropped to the bumpstops unless I've told it to.

Unless I'm going to be doing a lot of work and just need to get under it to have a look or do a quick job, I just put the suspension on high and get under there. The suspension has never just dropped to the bumpstops while parked any other time so why would it do it while I'm under it? Worse case, the body will drop but unless the tyres suddenly go flat at the same time, the axles will still be the same height above the floor. If you are really worried about it dropping on you, put the suspension on high then put a jack under the towbar and another under the crossmember just in front of the radiator.