The clamping notice must be suitably visible and must contain the details of who to contact in the event of clamping and the costs.

Some clampers are absolute scum. Mate of mines old man works for Siemens and was parked to do a job and his van was clamped. Like you said he was told the release fee was £150. After arguing for a short while ws then told it would be an additional £150 to cancel the tow truck.

The clamping company are now being sued by Siemens for the clamping and causing damage to the van whilst towing it away.