Thanks Onkel - I had a feeling in would be bad news (but cheap as you say).
I do have a backup but I don't know if it was from the right time (the one with this receiving address). To try it would I: stop bitcoin-qt, copy the wallet.dat from the old bitcoin folder to the new one and restart bitcoin-qt? If this is the correct procedure would I then be able to see the receiving address somewhere?a
yes, if you have a backup copy of your wallet.dat just throw it in the /roaming folder and let the qt sync up.
i think all your transactions will be there with your correct balance.