Getting Private Parking Tickets cancelled (DON'T PAY!)

[Kingy]

Ironically I'm now in the process of fighting an invoice from Excel Parking on behalf of my sister in law. She didn't pay the 50p charge (long story) and so they are now demanding £100 or £60 within 14 days. This is where I think it's wrong. If they asked for £10 I think thats ok and would have no hesitation in telling her to pay it for being a pillock. Still a big amount compared with a 50p ticket, but imho reasonable. £100 or £60 are not.
So, the battle begins.