@Maud132 I would just keep calling and reopening the complaint, refusing to hang up and requesting escalation to higher levels of management. I recently had a similar problem after a CS rep mistakenly applied a travel coupon to a deactivated account so I couldn't use it. Eventually I got through to someone who was willing and able to re-apply it to the correct account, but it took some effort. Posting on their Facebook page and tweeting at them may also help get the attention of the right people.
Earlier this year I sued Airbnb in small claims court for clawed-back payouts, and prevailed. I don't know if you have a similar option in the UK but if you do and feel strongly enough about the issue, that might be another option.