Well the plot thickens, as said my original measurements were 740mm for the middle mount, I've had 720mm told by a few people now so built a jig and offered it up and it didn't match the pictures of the undersides I have.

Cut a long story short I've just had a great conversation with Gordon Vincent himself, he measured the jig he has that has made all the genuine ones back in the late 90's, the measurement he has is 730mm which I now know is correct, so little bit of info to add to the thread.