if it's faulty they have no option but to take it back whether they have a sign or not. even if they didn't have to who would care? no matter who takes it back it'll be going back to the club/supplier

Take it back and if they get shirty (lol...sorry) just quote the Sale of Goods Act 1979 (as amended) and say that it is your statutory right to expect the goods to be of merchandisable/reasonable quality (which obviously it isn't in this case) and you want a replacement or refund.
If you know roughly when you bought it check your bank or credit card statement if you don't have the actual receipt (again not a legal requirement either!)

so if they dont have a reciept whats to say they didnt buy it from jjb?

correct,they will not take it back without a reciept.

As I said, evidence of purchase using your bank/credit card statement aligned to that period would be suffice, by law you are not required to produce an actual receipt of sale but do have to have some evidence of purchase from them (hence the bank credit card statement)
The club would then be able to check their own records if so wished or if you as the consumer were insistant. Given the specifics of a shirt with particular letters on a particular day it wouldn't be too hard to trace it on an electronic system.

And if you paid by cash?

The woman in the Saville Street shop said to me that if I had the credit card statement that would be fine. Problem is, it was paid for in cash and lost the receipt!

My shirt is like brand new, the only problem I have with my shirt is it does not fit me....gained a few pounds since I stopped smoking in Dec

Took mine back the day before the derby withoput a receipt / statement and they swapped it no problem. Went for a home shirt instead so I won't have to worry about it happening again.

Obviously I was answering in response to those saying you needed a receipt to take goods back.

However if you haven't got any proof of purchase and you paid cash, then that complicates matters but if you know what date and roughly what time you did purchase then the club should be able to trace it down on their system.
If you were particularly insistant that you bought it there then whoever is in charge should track down the transaction especially if it had specifics in terms of lettering which was mentioned.
Hope that helps you sort things.