I've just been on the site. I'm with Meds on this one.
My understanding is you have to place a weekly £5 bet (accie > 5/1).
This then qualifies you for another £5 bet, not a refund in the true sense of the word. The additional £5 bet has to be spent in Skybet, and not taken out of the account. So in effect you are getting 2 £5 bets for the price of one.
Or have I missed something bleedingly obvious!