Thank you OP, have now got the £10 credit to my account.
It did not work for me as well when I paid for the deal yesterday .. I have contacted customer service and they have credited my account with £10.
If it doesn't work you have to contact their customer service for them to correct it.
The link does take you to a page where the £10 credit is offered. However once i signed up it did not add the free credit automatically. Is there some sort of code that need to enter to get the free credit? The other codes specified under this deal does not work now as they might have expired.
This deal is still going on if anyone is interested.