Many thanks chaps,
Picking the bits out of those it would seem that the consensus would be to change the wheels and tyres in November, as soon as we start getting close to frost at night.
In previous years I've just used the 18" alloys with original specification Continental tyres all year round, and that has been a bit precarious at times (mostly in Northamptonshire and Norfolk). This year I found a set of Audi 16" steel wheels with good Yokohama winter tyres fitted in the summer, and used them for a week whilst I had the alloys refurbished and powder coated. So, I know the 16" wheels and tyres are fine, and I'd quite like to keep my pristine powder coated 18" wheels (I think they are called 'trilogy') looking that way.
That has been good guidance, many thanks - I'll look to swapping next month, until about March I should think.
Alan