I have had a 'ticking' noise coming from my P38 for some time now. First of all I thought it could be tappets or cam-chain and was dreading it. Last night however, when I started the motor, it sounded like a tin of bolts being thrown about and the 'alternator fault' light flashed on & off (+ power steering was 'jerky'). On checking this morning, the fan-belt was extremely loose which made me feel better (how expensive can a fan belt be) - however as the shops don't open until tomorrow, I thought I'd have a tinker about and eventually I removed the top bolt from the fan-belt tensioner ram. I then tried to move the ram and suddenly it sprung to the left and put the loose fan-belt under tension. After I replaced the bolt, I sprayed the top of the 'ram' with WD40 and started it up to see if anything had changed. Well, the fan-belt now appears tight, the ticking sound that I thought was maybe the cam-chain has disappeared and all appears to be working better and sounding better than it ever has done. I think I will replace the belt tensioner ram (as if it seized once, it is liable to do so again), but as it is a 5 minute job, I am happy to do that (£30 on e-bay). I don't know if this is the same problem as your one, but give it a try & if it helps, you'll get away with a very cheap fix. Also, although I now live in Spain, I used to live in Seaford, East Sussex - I had a 'classic' at the time and I also used 'terrain vehicles' > they were very helpful, but a bit pricey - but these trucks are expensive to work on, so I suppose we have to put up with these prices. If I remember right though, they used to keep a number of scrapped vehicles for cannibalization - surely they could have used some parts from these vehicles.
Also, when I replace the tensioner ram, I am going to have to move the tensioner back > any ideas on the best way to do this - I can see a hole in the center of the tensioner which a bolt head fits into - does this mean I have to get some special tool to move the tensioner back or can it be done some other way (crowbar perhaps) only their appears to be very little room to move in there.
Also, My air-con doesn't work & I have never used it anyway - so while I was doing the above, I removed the air-con belt > is this liable to help the fuel consumption or not.