thankyou for your reply Cooker, it's apprecaited.

I am just about going mad. It doesn't matter what I do, how I try to put it on, on part is too small and won't fit on and the other seems to big and doesn't 'latch' on. The locking lever makes no difference.

Also when I press down on the valve, even quite lighty, is just sink back into the wheel and I have to pull it out again!!!