I second the Pepipoo info.
Private companies cannot fine you. All they can do is seek recovery of any of their losses. I had the same thing with a company called Parking Eye. Best course of action is to just ignore it. They gave up after a few begging letters, because they knew their chance of winning in court was zero. For information, it was for supposedly overstaying in a free car park. Their photos were unclear, and I do allow someone else use of my car at times, therefore I've no idea who was driving.
If they did take it to court, they would have to prove their losses, not anything that would be considered a penalty. In a free car park, their loss would be £0. Hence the no-court thing, it would cost them more to pursue.
The private companies have done court, and shockingly have won cases. However, these are usually won by their claims not being defended against, or the person taken to court contradicting their evidence. These awards tended to be done by judges who unfortunately didn't understand, and were always overturned on appeal.