You posted: 'I do Private Hire part time, the vehicle wouldn't have been parked and left there, it would have been to pick up a passenger from the location.'
..is not a defence. Being there stationary for more than a moment is all that the money-grubbers need IF the location sits within a private area of land subject to parking controls which have been brought to a driver's attention.
As you do part-time private hire work then as you received your first notice 5 weeks after the PCN was issued then surely you would still have had your logs which would tell you whether you knew who was driving. But putting this to one side for the moment..
The car was at the location in their photos. So, side-stepping threats to involve the ICO (who wouldn't get involved in the substantive issues anyway) why not go to the location and suss it out. IMO this would be time well spent. If it throws up a potential line of defence, good. If not then everything hangs on the PCN(although as the creditor is IPC-affiliated you wouldn't succeed at this level of appeal and would be relying on a court.
GSV is not current. But what does arise is that the car was parked on what to all intents and purposes is a road, it's even got public realm traffic signs.
IMO, while we're waiting for their response go to the location, see how a driver could approach from the nearest public road and see what entrance signs are displayed, see this:
https://irp.cdn-website.com/262226a6/files/uploaded/Code_of_Practice_v8.pdfPages 20, 28-30.