Hello folks. well I have a VFR400cc NC30 and just found out a rear tyre cost the earth so what I would like help for is? can I change the single swing arm wheel which is 18in to the normal 17in wheel any help would be much appreciated by a student mental health nurse which is not on a lot of money. thanks you all

Yep, there are loads of threads on it, do a search for 17 conversion.
An NC35 rear wheel (17") include the 35 linkage and you're good to go, pretty much.
Also, you should be able to get a set fitted for the 30 for around £215, that's what I've been quoted anyway.