If the legal representative of a company suggests that their client had no knowledge that they had entered into a contract (even though it bears the signature of their MD) due to what they call a "Genuine Mistaken Belief" Is that a valid argument for a lawyer to make under English Law?
A belief that one is not signing a contract and are instead signing something else (e.g. an acknowledgement of receipt of a contractual offer) would be a valid defense because in that circumstance, the signature on the document wouldn't constitute an acceptance.
Now, if the document says "Contract" or "Agreement" on the top and has a sentence stating "We hereby agree to the terms above on behalf of XYZ corporation", this argument might be unlikely to succeed. If the belief that the document was not a contract was unreasonable, it would not prevail even if the person signing subjectively thought that it was not a contract.
But, it would still be a valid argument to make in a contract dispute and could prevail if there was both a subjective belief that a contract was not being entered into (a "genuine mistaken belief") and that belief was reasonable under the circumstances as they appeared to the person signing it.
Also, a signature obtained under false pretenses (e.g. telling a person with poor vision that it is something other than it really is) is considered "fraud in factum" and is void from the outset. Honestly, fraud in factum is probably more common than accidentally signing a contract believing it to be not a contract.