I think that my "problem" is not really a problem.
The bottom of the bellows is holding fast but it's the upper that is not.
I now understand that the upper is not supposed to be fastened and is supposed to float. However I think the bellows has been compressed for so long that when I raise the head up to the 35mm position the bellows does not come up with it and stays down, making for quite a big gap.
I'll look through the threads mentioned above but may have to put my bellows on some sort of stretching program so that it will do what it's supposed to.
Thanks for all the replies.