Surprised none of our Brit buddies have weighed in on this one. My first inclination is to ask where you bought your bike. Unless you bought from an owner they should have been an authorized dealer?
In all seriousness with minimal handtools and one specialty tool you can do the job yourself in about 2 hours tops. There is a thread by Joe Bulva where he describes changing the variator to a JCosta on a 250. Excellent photos that apply to all models on how to get the thing apart. There are several other threads detailing the Dr. Pulley sliders and the Fuzzy Washer. The specialty tool you'll need is the Buzzetti variator tool available at many locations/mail order. It's very affordable - especially since when you learn how to do this mod you can do your own belt maintenance and save more than a few quid. JimC, the moderator for this group should not only be able to help you with all this - he also hosts the user and shop manuals for your scoot.
I can supply the fuzzy washers - I just got what I plan to be the last batch and there are some left. Look for the thread on how to order if you'd like one or two. As far as the Dr Pulley slider weights (highly recommended by the way) I started with the 16 gm weights and felt the bike revved a bit too freely. I changed to the 17 gm weights and I'm much happier with the overall response. You can order these direct from the manufacturer or they may be available locally at a shop. I'm pretty sure the owner's email is
paul@unionmaterials.com. He is a supporter of this site and his user name is DRPulley - a quick search can find gobs of info on that subject.
Welcome and safe riding!