Pedantic point:
It's not a 'fine'; it's a 'parking charge'. (Private bodies can't fine people).
There are plenty of people around here who will tell you that you can ignore it and nothing will happen. That may well be the case sometimes but certainly not always; the parking company has the right to pursue the debt through the courts, which can add considerably to the costs for the motorist:
http://www.rochdaleonline.co.uk/news-features/2/community/15278/judg
There may also be others who will tell you that it's only the driver who can be charged for excess parking, not the registered keeper of the vehicle, so simply refusing to disclose the identity of the driver will suffice to avoid payment. That was indeed the case up until the Protection of Freedoms Act 2012 was passed. However that Act banned wheel-clamping but recognised that those responsible for parking controls need some form of enforcement measures, so (in the absence of the name of the driver) the registered keeper of a vehicle can now be pursued through the courts for parking charges.
So you can take a gamble and simply ignore the parking charge (risking the possibility of expensive court action) or simply pay up. It's your choice.