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 wsn't licensed
Different matter now
Criminal offcence on his part max �5000 fine
Worth checking
Got my money bask, so sheck your receipt
Other bits are clear signage etc
Visit the above site
Good luck