Many thanks to you both for your responses.  The links are very useful and I have reviewed the information.  With Christmas in the way I've left it too late to write a considered response, so have decided to pay up, but I will have the upper hand by never using their car park again. So they lose longer term.
Thanks and best wishes

Hi, I've been 'invoiced' by APCOA^▸ (Car parking company used at GWR^▸ (Great Western Railway) - controlled stations in the area) for failing to buy a ticket at Silco Drive on Friday. I'm in the wrong, I forgot to phone in - it's age related!
I've read another post, but the replies were for an unfair invoice where the enquirer had paid.  My question is, what's the situation re payment of the fine?  It seems an excessive amount for a genuine error.  I'm a repeat customer and registered on their phone-in system.  Can I make a reasonable offer?  Or question the work permit of the parking attendant? Does anybody have any experience to share?  Or do I need to accept it was an 'out of luck' Friday?
Thanks for your help.