you can get aftermarket from china for about $10US each plus delivery, just buy a few different style/colours to justify the postage cost.
try www.gowilddesigns.webs.com they sell them for $25AU but i'm sure postage to the UK would be reasonable
or you could try ebay. most don't advertise NSR screens but if you ask someone selling a CBR250 screen they will be able
to get an NSR screen for the same money.