Fixed Penalty Notice Advice?

I was issued a fixed penalty notice on the 9th June. I am hacked off getting it in the first place as my SORN car was parked in a car park I thought was private for residents. Turns out there are some weird and wacky rules about what counts as public road. I have 21 days to pay it. I have phoned the office today for the third time only to be told to phone back again another day because the notice hasn't been posted on their system. If it hasn't been registered as yet and I don't pay it within the 21 days will they penalise me further? Has anyone had a situation like this?