What do you mean by 'please complete this'?

Hi Received letter from DRP (Debt Recovery Plus) to pay £160 for unpaid PCN from 14.4.18. I've never received a PCN from UK Car Park Management Ltd before, so this is the 1st time I've seen anything! Just called DRP to get more info - they have photos, say I exceeded maximum stay (or may not have paid). I remember being surprised on the day that it has changed at Brighton Met College to the new system, lots of people that day had confusion, couldn't pay by phone (there's no cash option) or was told carpark full, which it clearly wasn't. I have phone record on that day, but that's all but think we may have had similar situation where it said full and we put notice in the windscreen to say this. They asked if I moved recently - I did in Oct, but did update my address with DVLA, etc. Again this is the 1st I'm hearing from them. They just said on the phone I have no right to appeal or dispute, to pay £160 or further action! Any advice would be appreciated - should I ignore, should I contact UK Car Park Management directly instead...? Many thanks,