Write formally to Simply AND to Capital One saying that you hold them jointly and severally liable and that if the money is not refunded to you and in your account within 14 days you will take out an action against both of them in the County Court for the value of the card, all postage and packing, telephone and court costs (tell them how much this is). Because the value was above £100 then Capital One must pay you if Simply do not.
If the money doesn't arrive in 14 days, take out a court action and Simply will have to pay all the costs. Even if they just repay the £189 they will not have discharged their liability.
Normally the threat, particularly when also directed at the credit card company, is enough.