Has anybody any experience or knowledge of genetics regarding overshot mouths,should one not keep a puppy from that litter to show and potentically breed from ,if one of the litter of 9 has an overshot mouth,neither parents do themselves,and also both have excellent hip scores

I don't know enough about the genetics of what you are asking to reply in any detail but I do know that incorrect mouths can be very difficult to breed out.

Can you see the grandparents and any other siblings of either to see how often it has cropped up.  If the pup you have chosen appears to meet the breed standard in all other respects then I would be tempted to keep it as you can't know where the overshot mouth has come from, just maybe avoid a close breeding in the future.

Think it also depends on how strong the breed is. In some numerically strong breeds it may not be advisable, but in some smaller breeds it might be a shame to narrow an already small gene pool even further just by trying to avoid a fault which might not crop up again for generations.

A lot of puppies who appear over shot at a young age have perfect teeth after the teeth have changed as the bottom Jaw grows. 

I guess you have to evaluate how good the dog is and where in your list of criteria does the bite come.  If it is not a problem commonly known to that particular line then it may not be as quite a concern but if it is cropping up often then you would need to try and eliminate it.