I thought I needed to bring this thread up to date.
Basically the engine still starts cold on 3 cylinders, and it's definitely No3 which isn't firing when cold. After the engine has been running for 5 or so minutes, it suddenly starts to run on all 4, so there's something amiss, and I believe this fault was the reason for all the work and damage, before I bought it. I can hear a chuffing sound from the exhaust when it's running on 3, so I'm thinking that maybe there's an exhaust valve issue, which resolves itself as the valve heats up, but this is just a guess, based on this chuffing sound.
For now I'm happy to live with this, as it's actually not as inconvenient as it would seem. It only runs on 3 at idle, and starts running on all 4, if the vehicle is driven.
The heavy regular misfire under load was due to No2 leaking around the copper sealing washer, which stopped when it was replaced. However I'm still getting a more random misfiring under load above 2500 RPM, (or was before the rear diff grenaded itself)
, but I'm thinking this could well be down to bad copper seals or crud on the seal faces. I've got an injector seat cleaner, so will be cleaning the seats properly, and installing all new copper washers.
I'm hoping this will cure the misfire, or at least give me a good chance to eliminate the seals as the cause.
This weekend I plan on getting the rear diff out, and hopefully be able to repair it. More on that on the relevant thread.