Have you checked the voltage at idle (yeah, I know you're struggling to get it to idle) from the MAF at the ECU?
Should be 1.4v at pin 16 (UG) on the large connector

I would, but mine runs as rich as a rich thing on petrol idle. It's OK once it clears its throat but that won't help you.
I'm on Tornado chips, single point lpg and start and run it only on lpg so the petrol just happily circulates from the tank to the motor and back again.
When I get bored and fancy a fiddle, I've swapped lambda's and looms, MAFs, ECUs and I still get 7 volts out of my Titanias (theoretically impossible!).
Actually, I'm not sure if there's anyone on here with a GEMS that's running cleanly on petrol. Let's see...

Yep- they can be frustrating little minxes at times, but you did buy it for £360 as a non-runner so you're just about up to market price now.
That's not meant to be a smug p!ss take, by the way, just an attempt to put your troubles into perspective.
These cars were and are complex for their time. Now they're old, complex and tired. Yours and mine are 20 plus years old. Mine's already circled the earth 5 times, Gilbertd's around 12 times.
From my experience of rebuilding cars and bikes twice as old as these, anytime you touch something, something else will be disturbed that has sat there getting crunchy and decomposing since it was built. Anything electrical (cables and connectors) will cause pain, Anything flexible will break. Any seal will probably leak. Any fatigued metal will crack. That's just the way it is.
Your car's not turning out to be a "quick fix", which is frustrating and a bit of a kick in the ego. We've all been there and share your pain. It's time to have a cuppa or something stronger and walk away from it for a bit.
You know your way around a set of spanners, you have the right diagnostics tools, you understand how these things work and you have knowledgeable people around that you can call on, so you're not in a bad place. You've just got to methodically keep on the way that you have- identifying issues and dealing with them, one by one.
Stick with it. It is only a collection of a few thousand parts loosely connected, after all!

14.5mm on the standard Bosch. 15mm on the GEMS

"...I still live in fear of something horrific stopping me (and more importantly my family) miles from home..."

AA Relay is your friend. I have the personal, rather than vehicle, cover for self and family members. Vehicle cover is hard to get if you drive anything over 15 years old.
Pretty good price using Tesco vouchers or whatever the missus wangles each renewal.
Used Relay for a 300 mile lift in my Hybrid, 200 in my SAAB, both terminal. Takes a while as they have to swap flatbeds each time they get to the limit of their patch, but a great way to save fuel :-)

Took the plunge and bought a set of the H4 LED 4000 ones from Powerful UK as I can't live through another winter with the crappy dip beam (even with my currently fitted Nightbreakers).
While I was at it bought a set of Nightbreaker H1's for the inner main, even though didn't really need them as mains, in combination with my KC Daylighters, were already pretty good. It's a shame your night vision goes to sh!t as you get old!
Will report back on the results once I've got round to fitting. That might take a while as I've got to replace a headlight wiper motor at the same time, which will mean repairing the fixings for the brush guard as well...

Me too. I've got a couple of the early single speaker subs and they also feature the sealant round the cable hole

There's a good section in Des Hammills "How to tune...." book that explains the different crank types in detail:
https://books.google.co.uk/books?id=gmSavy1RPxEC&lpg=PA22&ots=nh6x9ydopZ&dq=rover%20v8%20long%20nose%20crank&pg=PA21#v=onepage&q=rover%20v8%20long%20nose%20crank&f=false
In a nutshell, as Gilbertd says, all of "our" 4.0 and 4.6 P38s had long nose cranks

Sounds like a bit of a juggling act stuffing it all in the back of the RR. Sort of reminiscent of moving a 3 wheeled Disco around a car park on an engine crane :-)
A 6'x4' general purpose trailer is only about £18/ day to hire. You'd get the engine and the crane into that with much less risk to interior and your back!

I still have a couple of decisions to make... the most important being - do I get the new engine delivered to home, or to the workshop... in some ways working on it at home would be easier, but then I'd have to lift it somehow into the car, when it's nearly assembled... whereas at the workshop, I'll have the crane already there and I can hoist it off the palette, straight onto the engine stand. The down side to that, is I have to head up to the workshop, whenever I want to work on the engine - which requires more planning regarding parts etc - and also more miles on the current leaky engine..

I don't know what sort of access you have to your home workshop ( paved access from road to workshop?), but if you have a good strong engine stand with castors and can manoeuvre your engine crane around in there, I'd go for building it at home.
Good lighting and operating theatre cleanliness are a great help, although you can do the job anywhere.
You can hire a small box trailer to move the nearly built engine from home to workshop. All you have to do is shuttle your engine crane and stand between the two.

At a guess, the semi circular and circular lumps are there to affect the "swirl" of gases inside the combustion chamber.
That'd be my technical guess, but more likely the semi-circular bit just supports/ covers the thread at the end of the spark plug :-)

Morat wrote:

What's that funny bump outboard of the spark plug? I've no idea what it's for...

You can't beat a brass brush on a Dremel!

Morat wrote:

A bit of degreaser and some dremel work would have those bits nice and shiny, for sure :)

Time to get the dishwasher fired up to shift the last of the crud. I think you should have dropped the bits over to Morat so he could give them his trademark polish :-)
What cleaning solution have you left the bits soaking in? I use good old diesel on ally stuff as I'm not sure about prolonged exposure to the "eco-friendly" water soluble cleaners.
Those pits should be OK as long as they don't bridge the fire ring of the gasket. They look more like FOD than inclusions in the casting. Don't see how though?
Have a good work break...

Current research shows that a Hydrazine/ Methanol mix may be the way to go.....
:-)

Are you using a Nanocom Aragorn?
MAF readings should be in Kg/Hr:
Air mass flow at sea level - Engine fully warm, in neutral gear, with all loads off
At Idle 20 kg/hr ± 3 kg\hr ...............................................................................
At 2500 rev/min 61 kg\hr ± 3 kg\hr ................................................................
I'm guessing you've just scrambled your units otherwise, if your readings are actually grammes per second that equates to:
Idle- 72Kg/Hr
3000= 216Kg/Hr

gordonjcp wrote:

I don't think I've ever had more than a quarter tank in mine, and it seems to adapt the fuelling just fine...

Got that info straight from the horses mouth- well Mark Adams actually. I figure that if there's anyone who knows the intricacies of the GEMS ECU as well if not better than the guys that designed it, it's him!

Kind of implies that it'd mess with a sequential lpg system on a GEMS (rare as they are) under the same petrol in the tank condition.
I've only just got my head fully around the intricacies of single point so I could be completely off the mark, but if the trims are locked off when low fuelled, then petrol injector durations will be potentially incorrect. If multipoint ECM gets its lpg injector timings from the petrol injectors and then applies calibration mumbo-jumbo to the duration of the lpg injectors then you'd get messed up mixtures on lpg due to locked off petrol trims.
Happy to be called out on that though! As I said, only just got my head fully around single-point and all I know about multi-point has come from a cursory study of how it works from the web...

The GEMS will not adapt the fuelling if there is 1/4 tank or less. This is to prevent false adaptions due to fuel surge. Large corrections will be held until the tank is properly filled- at least 1/2 tank.
Do you have plenty of fuel in the tank?

Hot is best (if you can get it warmed up) as pistons and rings are expanded.
Makes it more fun juggling hot plugs etc as well :-)

Did you get a chance to chuck a shot of oil into the low pots to see if the pressure increased?