[David Hodgson]

Slowly, this is exactly what Gaz was referring to in post #5. Most engine builders do away with the cork gasket and use a thick bead of an appropriate silicon. It fills the gap exactly where as the cork can sometimes be too loose. If you must use the gasket, half a dozen decent centre punch marks along the block where the gasket sits will help keep it in place.
The missing rear gasket may still be stuck to the block
Cheers