Signs must be displayed, if they are not you have a good chance of appealing. As for going out and clamping on your own private land ..... you cant just go ahead and do it, the advice you received from the Police was wrong, In order to carry out such activities you need to be licensed under the Security Industry Act (SIA) and it is to them that any infringements may be reported. The Police are not the Prosecuting Authority here it is Trading Standards so there isnt much point in complaining about the Police,they dont have any power to intervene unless another criminal offence is taking place, they will turn up but only to keep the peace, they will check badges of the clampers (its an offence not to wear one) but that's it. It is worth speaking to the landowner, as afterall the clampers operate on the landowners behalf, if the chemist new that the clampers were being so zealous they may withdraw their consent (this can be done retrospectively).