Update on this, ours has temporarily left us due to the owner needing to gat back to his Army base 150 miles away. We'll see it again next time he can spare it.
There is a garage in Essex with exactly the same issue, he has been working on his for a couple of weeks now & we have compared notes & between us tried different approaches.
Anyhow, this morning on the advice of Autologic, he slightly advanced the exhaust cam to a point where he was no longer able to insert the locking pin. Locked it off & it ran perfectly through the warm up phase. This info was gleaned by AL off an American website from a tech suffering the same issue on an early BKD engine.
Thus far it appears to have worked - when I get mine back, I'll give it a try & if successful post it as a fix - if not Fiachra, Dave, Phil - I'll work through some more tests.

Cheers so far,
Jon.