Pretty much answered my own questions whilst working on the bike today.

The 18's tubes are slightly different at the bottom and the damping rod is completely different.

So I decided I'd rebuild the set I took off my 21 when I fitted SP ones.

The 18 forks are 30mm ish longer than the 21's so have in effect dropped the front 30mm. Not sure how twitcy it's going to be but I suppose I'll find out on Tuesday.