Thanks guys!!
This engine has always suffered from excess crankcase pressure. It was designed as an APU for a British Army battle tank, where leaks were not a problem. There is not a single non-mechanical seal on the whole unit as it relies on a reverse archimedes screw thread on the crank ends to "wind" the oil back into the block.
Using a fast acting one-way valve from a brake line seems to be enough to prevent any reversion, but I've not had anywhere near as much "vac" as others have had. I've just tried running the end of the stub pipe further downstream into the tail of the "y" piece to the thinnest part and flared the end to further reduce the cross-section of the pipe. Should be able to fire it up again in ther next few days.
Fingers crossed!!