Well, it wasn't an air leak at all. Current suspicion is a dry LHS exhaust rocker. Running the engine with the rocker covers off I'm getting healthy dripos from both RHS rockers but less from the LHS inlet and hardly anything from the exhaust.
It could be a restriction in the feed via the 'U' pipe from inlet to exhaust but my suspicion is the rear portion of the external feed pipe has a restriction.
When I received it from Drags it had more than a restriction - it was completely blocked with solder (!) and I've had to take it apart, clean it out and re-solder. Maybe I've got a bit in the rear part of the 'T' that is restricting the flow to the LHS of the head. Time to take tank and carb off again to investigate so might as well deal with that thread while I'm at it!