i used to have one i bought of ebay (where they dont appear often, but sometimes they do)
never used it though... and it's been sold again
http://www.bike-teile.de/index.php?seite=einzel&teilid=278062here is a restrictor set, but it says its for 400, 600 and 1200 and it looks different from the one i had. give them a mail and ask them if your interested. this one looks like it should be used between the carbs and head, mine had to be used in the carbs so you couldn't drive full throttle.
€ 120 is alot cheaper then £ 200
i guess i shouldn't say i paid € 40 for my restricter set when i bought it and thats also what i sold it foror you can just make it yourself for less then € 5
use a few (plastic) rings in the carbs to limit full throttle (if i remember correctly mine were about 5mm thick) or drill some holes in round plates (like the link) and use them between the carbs and head.
or do a combination