matt, my wife has an account on will hill and she deposits using a matercard and hasnt had any bother cashing out ( not that i am aware anyway)
admittedly its not 600 quid shes cashing out its only 20s and 30s but i wouldnt htought the amount would make a difference
Below is some info taken direct from the WillHill site in the help section regarding Mastercard withdrawal
"Extra Info. 72hr suspension after 1st deposit to allow a security check
Mastercard (UK) - withdrawals sent by BACS direct to the card
Mastercard (Int.) - withdrawals must be made via bankwire or cheque"
It appears that according to WillHill my MBNA Mastercard falls into the catagory of International Mastercards (not sure how as all their details are UK based) so they are refusing to pay it back direct to the card.
It makes me laugh when they say about the account suspension
after the first deposit!! funny, never been suspended once making a deposit, just when i try to withdraw!!