Reading through the T&Cs on my latest contract, I notice the agency has included a nice clause saying that even if I don't sign the contract or verbally agree it, if I turn up to the client's site that automatically counts as acceptance.

Is this legally valid/enforceable? From past experience I have been able to work for a client whilst still resolving the finer details of the contract with the agency, and as there are a couple of points I would like to amend in the new contract, I'm obviously a little concerned about this clause.

Thanks