Same thing happened with me and Betdaq about a week ago.
A site I was on claimed that you got up to £20 refunded if your first bet lost. I opened an account, laid a horse at about evens, backed it on
Betfair and it won. Betdaq also had an offer on their front page - when you match your first £10 they give you £10.
I got my £10 no probs but when I asked for the refund of my £20 they said that the offer had ended and that their offer was now £10 and I had it. I kicked up a little bit of a stink and to be fair to them they compromised and gave me another £10.