Thanks for the informed highlights Simon, I'm sure that'll be of use here to those going for a P6
The variable delay unit is one of several little problem parts on the P6. The very last cars in 1976 had, I think, a different unit to all the earlier models. Some things to watch out for, with apologies if you know some or all of them already :
1) Top bearing in the steering column. Unobtainable new but can be got secondhand. They disintegrate completely.
2) Steering column adjustment bolt. Check it actually does stop the column moving up and down as it's quite a difficult little fix for something that looks straightforward. The plastic covers at the top of the steering column can go very brittle with age and sunlight.
3) Fuse box that has been on fire or got very hot. It's worth checking for melting or melted plastic.
4) Rust in the base unit at the bottom of the C panels. Check from underneath by putting your hand up as far as you can above and slightly in front of the rear wheels. If you find air or rusty bits coming away, be careful depending on the price.
5) Loose spare wheel mounting screw nut leading to rattles. This is a really awkward little fix due to the need to weld the nut back on in the boot with the petrol tank not far away - the sort of thing where a seller says "that's easily fixed, i just haven't got round to it..."
6) Clearances between the underside of the car and the exhaust. It needs quite a bit of patience to get the exhausts not to bump or rattle somewhere so worth testing over a few bumps and possibly with someone in the back.
7) Rust underneath the back seat. Take out the seat bases and check the base unit carefully, especially in the corners nearest the roadsides.
Rust in the sills. The cover panels give absolutely no indication of what the structural sills under them are like and they can look super on the outside and be virtually non existent on the inside.
9) The chromework. Those bumpers have an incredible amount of chromed surface area on them and with the 3500's overriders, they all need to be in really good order if you want to avoid a seriously heft rechroming bill. The steel underneath isn't the best and again, best if they've been well restored recently by someone else. Everything else is nearly all stainless fortunately but wants to be in good, non dented or twisted order.
My main experience has been with the 2200 so I've kept off the mechanical bits really here as you seem to be heading for a 3500. Just if it's any use, the manual gearchange mechanism, which I think is the same on both, now tends to suffer from worn bushes in the remote extension. these are £20 a pair and once you have the unit in your hands, they are changed in 10 minutes. However, it is a long and tiring job to reach that stage. Jack well up, remove exhaust and prop shaft, drop engine and gearbox, remove remote control (blind) from underneath, change bushes, reassemble. Sounds straightforward but the sort of job that is worth paying for someone else to have done it as there is a good six hours or so in it. It doesn't even rate a mention in any of the manuals because they never wore out first time round!!
The rear brakes are as awkward as they have a reputation for so again, worth paying for a recent comprehensive overhaul.
Parts for P6s have become far more expensive than they ever were in relation to some other cars. You'll no doubt know the usual suspects for parts but just be aware of the cost of things like ball joints bushes (of which there are many), shock absorbers and body panels. Heritage made some replacement wings a while ago and I bought two rear wings for the last P6 I had. The man I got them from ceased trading when he became terminally ill and his stock went to a well known supplier who was then selling the same wings for nearly 3 times the money. The point is, someone who has restored a car well five to ten years ago couldn't do the same now for the anything like the same money and so the best restored car you can find for the money is likely to be the best buy.
Finally, I've been told by people who had several P6s that the best of them were definitely the pre Leyland built cars. The earlier ones did seem to be less rusty and just better put together. I've always liked both Rovers and Triumphs but I decided last year to sell the P6 I had and have a 1947 Rover instead, as well as some Triumphs. They all have their good points and for travelling a distance in the P6 is a very satisfying car. Good luck!