Thanks for your thoughts people.
Tyres ordered - to be fitted on Saturday.
Guy on the phone mentioned checking the alignment when they fitted the tyres and said if there was any sort of problem they would sort it for £27. I'm always sceptical about things like that, especially as the current tyres seem to have worn evenly. Well, I say that, but there's both fronts have worn a bit more on the inner-most surface of the tread (c.f the outer-most surface of the tread).