Just had some bad experience with this myself
Before the Private Security Industry Act you could challenge clampnig on priovate land by claiming it iwas a trespass against your property
The Act then legalised it bot only if:
The offender never had the laondwoners permission to park there
The offnder knew he was liable by parking there
BUT, the clamper must be leicensed by the regulatory body www.the-sia.org.uk is that body
The conditions of this include clampers wearing phot id cards etc,
The receipt is where you may have a challenge, which l did
It Must contain the license number
Name and signature of the person taking the fee
Mine only had a name, so l chekced the on line register and bingo, he wasn't licensed
Different matter now
Criminal offcence on his part max �5000 fine
Worth checking
If the receipt hasn't got all the correct info on, tell them you will take it up with The SIA as they may get strruck off
Got my money back, �105.75, so check your receipt
Other bits are clear signage etc
Visit the above site
Good luck