I am running 18" alloys on my 2005 STi at the moment, if I get a puncture(hope I am not tempting fate) is it ok to use the 17" spacesaver spare that comes with the car or will I have to get an 18" spare.

Thanks for you answers

 

You will need to factor in the profile of the 18" tyre as well.
I looked at the size of my spare and it is a "funny" size.
I assume it has the same radius as the 225/45/17 tyre, but I am not sure.
I would be interested in an answer to this question as well.

 

Yep, should have said the tyre profile is 40mm

 

your space saver is matched to the 17s size. so will still be ok as long as you've matched the right tyre when changing to 18s i.e 225/40