XKR wheels for winter tyres (2010 X150)

[a8hex]

I was thinking of getting another set of wheels for my XKR to fit with winter tyres. I discussed the idea with the salesman when I bought the car an he said it was normal to use 18" wheels, but today while it was in for a service they said only 20" wheels would fit over the calipers.
Anyone know if the smaller wheels fit?
(OK, I want 18" wheels coz I'm being tight and a new set of 20" wheels is two and half grand)