This thread has got me worried now if this firm are sharks. I use a SIP carpark near work because its £3 for 12 hours which is about as low as you will get near Deansgate. It's pay and display with no cameras so i'm guessing your ticket was placed by one of their jobsworths.
My initial thoughts are:-
Has you son mentioned that he may have underpaid the parking bearing in mind his ticket would have been on the dashboard?
If your son denies parking there, do they have a photo of it? For all they know, there might be a stolen vehicle on false plates matching yours. The burden of proof is on them.
Putting the vehicle in your name for your son is fronting which brings down costs. Don't tell the insurer or they will void your policy. They will also ask questions if the named driver (your son) has an accident during working hours as they will probe the usage as a means to try and avoid meeting a claim. Just remember to have those explanations ready so not to get caught out. The reasons the rates are cheaper when fronting is because the claims ratios for young drivers is much higher and premiums simply reflect that. If they only have occasional use, at times when roads aren't busy, the risk of an accident falls which is why its cheaper doing it the way you have. The issue is if the insurer finds out, they will declare cover null and void for misrepresentation.
On another matter of parking, the operator of Lidl car parks have got on my nerves recently. My elderly parents parked up in Royton to do their weekly shop and they popped across the road for a doctors appointment afterwards. All expected to be within the 90 mins parking allocation. The docs were running late so they went over on the parking by 3 mins and copped a £90 fine which they paid before telling me. More annoying is that my mum was given a cancer diagnosis which contributed towards the delay and they still didn't see it fit to turn a blind eye over a 3 min parking indiscretion. I tweeted Lidl and scanned a copy of their £60 shopping receipt but they weren't arsed. They seem to have form for it.
Some of their other stores require you to scan your receipt on the way out of the store. You are fined for parking there if you don't. There is no facility to pay for their parking. I asked them what happens if you go in store for a single item that is out of stock. The response was buy something else or receive a £90 fine. At that point i decided to change my shopping habits.