Hi James,
You can fit an 18″ rim to a a Supa5. I did it a while ago for green laner and one of our Bristol section members has also done it. There were two snags as I recall. The first is the centre stand cross piece which will rub on the tyre if you get to ambition about tyre size and in the same vein, there is not a lot of width in the s/arm.
I got round the former problem by beating the cross tube flatter in the middle section.
If have one in my shed with a tyre fitted which you could borrow if you live close enough to pick it up (Nr Bath). My contact details are in the magazine (membership co-ord)
Regards
Peter