Loz, the 2V's don't have a head gasket as such. They use O-rings around the various oilways between the head and the barrel to seal.
I'd guess one of these has gone hard or got trapped on assembly.
It's not unknown, but isn't a regular failing.
The problem is that you can't get the rear head off with the motor in the frame
, so although the job itself is pretty straight forward, it's time consuming.
If you are going to do the job yourself, make sure the O-rings are greased up well and sat properly in their locations before tightening everything down.