I know that I have been asked to sign some kind of acknowledgement of an "early departure fee" at a couple different hotels in the past but I can't remember if St.Ermins was one or not.
If they are claiming you signed an acknowledgement of the fee when you checked in and you can't recall doing that, I would ask them to produce a signed copy. Even if you signed something at check-in that acknowledged the fee it should be clearly stated and obvious, especially if your reservation confirmation did not contain notice of the fee.
Assuming you paid with a credit card on file and did not sign a statement of charges including the fee at checkout, I would dispute the charge now and stop payment. Explain the situation to your credit card company and let them work it out. If there is a difference between the reservation confirmation and the check-in form, I would also call/email Marriott and tell that they failed to disclose this fee and you want the fee refunded.
Also if you paid on credit card, make sure to check the FX on the transaction. They may have forced a "pay in your own currency" charge at a much higher FX rate without your consent (this has happened to me at other London Marriotts). If they have, I would hold that over their head and threaten to report them to Marriott and the UK Fraud authorities. That should be more than enough to get them to remove the charge.