Well, I can now update this thread... after an embarrassingly long time!!
I sourced a replacement gearbox from someone who broke a P38 locally over 2 years ago! I got it fitted not long after I got it but it took me ages to finish putting it back together. When I started filling it with gear oil I noticed it was pissing out of the oil cooler so I think I found the reason the original box failed. The gearbox oil cooler pipes were rotten too.
I replaced both pipes and the oil cooler and had my Dad sit inside it moving it through the gears whilst I was under it filling the gearbox. He moved the steering from one side to the other and one of the metal power steering pipes exploded. It was crusty as hell and rotten too. I replaced the steering pipe from the pump to the box and bled that out. Thankfully whilst filling the gearbox I could hear the engine note change and could see the propshaft starting to take up the slack in the drivetrain.
When I thought I was finished I put it in drive and the car wouldn't move. I nudged it into Low and tried again, and with about 1500 revs it moved grudgingly. I stripped the brakes down, the drivers side front caliper was seized solid. I couldn't get one of the pistons to move even when using the airline behind it. the passengers side front was ok as was the passengers side rear. The drivers side rear caliper was ok but one of sliders was seized solid in the carrier.
I ordered a caliper, carrier and front and rear pads from Island 4x4. The discs front and rear were like new but the friction material came off the rear pads on removal and one of the springs fell off the front pads.
Once I'd done this I took the car for a short run up the lane and it was driving nicely and changing gear perfectly which was a relief! After a quick prod around with a screwdriver I found it needed a small patch of welding.
Today it went in for an MOT... It failed...
Both rear indicators were no longer orange... 2 new bulbs cured that...
Windscreen washers not scooshing enough fluid... This was harsh as 3 of the 4 jets were perfect and the 4th was hitting the bottom of the screen.
And the brakes... the efficiency was below 50%...
I trailered it to it's MOT as I hadn't had time to insure it so the new pads on the cleaned discs hadn't actually done anything. It had a run around the block in which we did some emergency braking and the brakes started to feel much better. I let the tester try it again and he got much better readings on the tapley meter and he compared them to my P38 that I'd gone down in and he was happy that they were ok.
So... 2.5 years after I bought it, the DSE now has an MOT!! :)
I started to do a bit of looking through it and found that although it has a sat nav screen in the front it doesn't have a sat nav unit in the boot... It doesn't even have the slot in the trim where it should sit!
The tester did advise me today that there is no play in anything when the car is on the ramp but when driving it to test the brakes he said the car wanders on the road and you're steering constantly to keep it in a straight line so it probably needs a set of bushes.
I'll collect it on Monday and see what it drives like.