AC compressor runs fine producing 37F air
Blower speed is on high and I can’t change speed on head unit from high even by rotating dial and getting low speed 1 bar symbol on display
Stopped vehicle and pulled out fan and compressor relays
Found fan relay was black and slightly melted
I put a new relay in and it got hot as a pistol fairly quickly then removed it
Any ideas where I should start troubleshooting?

If your blower(s) are running on full speed all the time, without being controlled by the HEVAC, then chances are the speed controller transistors in the blower motor itself have failed and they're driving the blower at full pelt.

That would be my first guess anyway.

Fusebox where the relay plugs in. Known failure are they not?

Blowers running full speed isn't going to be a fusebox problem, although the pins in RL7 socket burning up is another common issue but unrelated. The tracks on the PCB are in my opinion underrated for the current of the blower at high speed.

As Marty said, it's more likely the speed controller on the blowers.

The condenser fans can be pretty much ignored - the viscous fan moves so much air even when the coupling is unlocked and its 'free spinning' that the pressure in the high side of the system is kept in check. The fans are controlled by the pressure switches - the single pressure switch when shorted (reaching 12 bar or something like that) turns them on at low speed. At 15-17 bar, part of the trinary switch will change them to high speed. At 30 bar, it cuts the compressor off to stop kaboom occurring.

The usual culprits would be the fan speed control transistors and blend motors... though if the speed control, blend flap adjustment and displayed temperature have all failed at the same time... it might be an issue with the HEVAC itself. Maybe...

Is the chequebook making itself known on the display?

Yes the book symbol with exclamation mark is there.
I swapped the HEVAC control head with another vehicle to troubleshoot and see if that would resolve the issue. No luck.
Blend motors work fine. HEVAC control panel can control heat and AC. Issue of fan speed staying on in a continuous non controllable speed is still there regardless of input from either the original or swapped control unit.
I did notice that Relay R6 was showing signs of melting on it’s top and sides. I replaced it with another relay and the new one began to get hot as a pistol. Electricity is not my chosen subject but I assume that there is a problem here. Issue is what would make two different relays in position R6 stay shut or overheat?
Any ideas?

Update.
With the R6 relay in the fans blow high once I put the key in ignition and I have no control of its speed from HEVAC control unit. When I pull the R6 relay the HEVAC controller can control fan speed, although at a lower volume of air due to only one fan operating.
I swapped the fuse/relay box located under the bonnet out with another unit and the same occurs.
Do I need to replace the fan or is there a component that needs attention? Which side fan does the R6 relay control?

Relay 6 controls the Right blower motor, passenger side in your case, so it is the Left hand one that has died. Simple enough to confirm if you pull the pollen filters out you can see the blowers down the hole and see which one is moving and which one isn't. With only one blower working you will get much less air, less than half, because normally both blowers are shoving air towards the heater box and it has nowhere else to go other than through it. With one blower not working, some air will go though the heater box while the rest will just go out via the non-working blower. If you poke the recirculation button, that will close flaps between the pollen filters and the blowers and at least keep the air in the car.

You've got the opposite to what I had at the weekend. One blower stopped (due to a dead spot on the commutator), the Hevac detected the fault so cut the power to both blowers. Not fun on the first decently hot day we've had this year. Fortunately, I had a couple of spare blowers kicking around so changed the dodgy one and all back to normal.

Hmmm, are you sure that actually, its not just the left hand fan running full pelt the whole time (likely due to a failed speed controller onboard), and the right hand fan IS actually adjusting, but it just doesn't sound like it over the noise of the other one? That seems much more likely to me.

Press A/C off and turn the speed all the way down - I would expect the left one to keep going as usual and the right one to slow down and stop (it might be blown around by the airflow from the other one as above). I can't remember if on a P38 the blowers are still visible from the pollen cover aperture when recirc is on or not - but if they are that would stop the air being blown out through the stopped blower and confirm/raise more questions about what I'm suggesting above.

No, the blowers aren't visible through the pollen filter hole if the recirculate flaps are closed.

@Gilbertd....you wrote the following ‘Relay 6 controls the Right blower motor, passenger side in your case, so it is the Left hand one that has died. ‘
This has me confused a little bit.... can you confirm that Relay 6 controls the right blower motor?
Your message suggests that it’s left hand one that has died

You are right, I am confused, relay 6 controls the RH blower so if it behaves itself with that out, then it is that one at fault. Blame it on the heat, we aren't used to it being hot two days in a row in the UK.

Haha.... understood.
Removal of Right Blower motor it is....
Hoping that a cleaning of the contacts etc will resolve the issue...

Check by seeing which one isn't moving before pulling it out. Having just taken one apart to see if I could nick the brushes to go in a very similar Valeo blower in t'other half's Merc, you'll be very lucky getting it apart. It can be done but.......

I found stripping motor quite easy. I just used a wide blade sturdy screwdriver to lever the blower out on the three rubber bits. They were stuck but do release :)

One thing I discovered last night (or very early this morning to be precise) is that the brush guides on the Valeo motors are a hard plastic. If the brushes aren't making good contact they get hot and that allows the plastic guide to form a ridge so the brushes are no longer touching the commutator. I got the brushes out of a noisy P38 blower motor and found they were no better than the ones in the Merc motor I had in bits. Managed to clean up the guides, sand the original brushes down and it now works perfectly.

However, in James's case, I suspect it is the speed controller that has died rather than the motor.

@Gilbertd, can the speed controller be swapped out easily enough or am i only prolonging the inevitable?

@Gilbertd, can the speed controller be swapped out easily enough or am i only prolonging the inevitable?

Don't see why not. Prise the motor up out of the housing and unscrew it. That's the only reason I've kept the one I butchered the other evening, I know the controller was still working so while the motor may be shot, that bit is still worth keeping as a spare.