Free Traffic Legal Advice

Hi everyone, Im hoping for some advice please. I've received a letter from DVLA fining me for a vehicle that is registered as SORN and parked off the road in a council owned residential car park. The car park is not tarmac and does not have bays so I'm unsure where I stand. I honestly thought I was parked legitimately off road. The vehicle has MOT and currently listed for sale. Sorry if most of this information is irrelevant, I'm just at a loss on why my vehicle has received a charged and a neighbours car (also SORN) hasn't.

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