Russell was famous for his debating, with his speeches and writing readily available. What would engaging further with Mosley have achieved?

A link posted upthread indicates the context was an initially polite exchange of not completely incompatible opinions on something related to foreign policy, followed by Mosley offering him lunch.

I shall have to remember Russell's turn of phrase as a way to turn down meetings I don't want :)

Indeed, and "what Mosley believed" was pretty well known at the time given his fascist activity over the preceding thirty years. Mosley was not likely to change his mind, and while there may well sometimes be joy and enlightenment in the practice of debate and rhetoric, you don't have to do it with a fascist. Bertrand Russell had nothing to prove and was perfectly reasonable in saying, effectively, that they were never going to agree and there's no point in wasting more paper in proving that.