All too common with this outfit unfortunately.
Is it really in the terms and conditions that you received?
If it is then unfortunately if it's also more than 14 days since you received that paperwork (not the day you arranged cover) then you are screwed.
If it's not then write to them (using a delivery method that gives you proof of receipt) demanding the money back in a reasonable amount of time (7 days from receipt of the letter should be perfectly reasonable). If you get no satisfactory response there involve the ombudsman and tell them you are doing it.
Buyer beware - especially of Flux.