Hi Colm
I got your private message. No, I'm not a solicitor. (even if I was my advice wouldn't be a 'legal opinion' as you're not my client)
You likely have a solicitor, you should ask him for advice. Clearly you need professional advice for such an expensive thing like a house sale.
I'd be surprised if a buyer could cancel on the grounds that he didn't receive a copy of the signed contract. Any buyer could easily lie about that.
Your solicitor would be going to court with your signed copy of the contract,... and your buyer would have the same contract. If the buyer loses the contract then he has no contract but is still bound, ... you can still enforce your copy.
I'd suggest you get a solicitor, and you ask him to ensure, with 100% certainity, that you are performing your side of the contract, and not to allow any reason for a pullout by the other side..