Hi James,
Thanks for the reply, never actually though to look at the manual, no ceramic brakes so planning to get 18's as you say more options and better for the winter anyway. I also found a converter online where you can compare wheel and tyre size options against your standard set up and it tells you if they advise it or not but again as you say as long as you are within a few % it'll be ok.
Cheers again,
Kev