Thanks Namsag, we will give that a try although we have tried putting water into the venting pipe in the loft and that does deliver water to the hot taps, so if there is an air lock it would have to be between the tank and the tee piece in the pipe running between the loft and the taps.
I have just helped my son replace his hot water tank, which is direct/vented, and has two immersions on economy 7. Having completed the job successfully, we can't get any water out of the hot taps. He tells me that they already had a problem that the taps ran dry when the hot was used up, but...