This is an off-street car park so there needs to be a tariff board listing the terms and conditions of parking, which there isn't according to the CEO's photos. That being the case this should have been an easy win, but unfortunately
@davehurd has paid therefore throwing his money away for nothing (no you cannot claim it back).
Dave, for future reference you should
always make an informal representation as the council will always reoffer the discount as long as a challenge is made during the initial 14 days, and doing so buys you several weeks more time to get proper advice.