All depends on the local Authority or the Private Car Parking Enforcement Company, most Councils will chase up in order to get paid, Private some will some wont, the problem arises is if they do go down the route of a Court Order for payment its all a slippery downhill slope from then on.
A few years back had a Private Ticket from a Company in Hull, about 40 quid from memory, ignored it and there follow up letters, 14 months later, Car was clamped outside my house at 6 am in the morning by a Certified Bailiff in Essex, cost me £550 in total.
Can't do clamping or towing away any more sadly, as our management company looked into it recently to enforce visitor parking spaces. As they used to physically stop/take it away, you had to stump up, now they are not allowed and are not enforceable. Renders all private ones pointless.
Council ones still can be enforced by the police, so have to contest them 
Trust me, a Bailiff with a Court Order for Monies Owed, can Clamp or Seize / Remove your Vehicle.
Yeah, but they need to get one of those first. They used to be able to clamp/tow away just for breaking the rules, before court. It's the hassle of going to court to get that right, all these companies rely on is mugs just paying up straight away.
If you read my post correctly, do you really think a Certified Bailiff is going to Clamp a Car outside my House at 6 am in the morning without a Court Order. 
Well I know that, but you missed my point.

5 years ago, you could park in a private car park, if they deemed you in breach of their rules, they could clamp you there and
then. No court order, no letters, no nothing.
Now they need one. Which is what I stated, so yes I did read it correctly
