Well finally solved the air in fuel problem on my 100k 2002 Defender 90 td5. Yep, injector seals. Took me 4 hours start to finish to change last Tuesday, been right as rain since. Spent a lot of time messing around with the air bleed valve and looking for leaks in pipes, regulator etc first as the problem didnt occur until the tank was around the half full mark so I didnt think it was the seals. Now I have sat back and had a think about it, the fuel pump sits half way down the tank in a recess. Correct me if I am wrong, but I think that what was happening is with a full tank, the fuel return is covered with fuel and holds the diesel in place in the system a little like when you hold your thumb over a siphon tube. Once your tank starts to empty, the return is uncovered and then, when you switch off, the faulty seal allows diesel to seep away pulling air through the fuel return causing the problem. Cylinder 1 copper seal was totally shot, pitted and burnt. The others were not too bad. Strangely, the cylinder balances looked ok on my Nanocom Evolution, why is that? Any thoughts folks?

Probably because as the engine warmed up the copper washers sealed properly. Once they cooled off they let air into the system.

At least you fixed the problem.