I live in the states and just got a Derby GPR about a week ago. Here is a link to an excellent UK derby sight http://www.geocities.com/gpr50r/
My Derby I bought here in the States was one that Derby built in a lot to be sold here for a National Race by ARMA called the Derbi Cup. There are several races held around the USA racing just these bikes. The first race was in March on the High Banks of Daytona florida on the same course the Daytona 200 motorcycle race is held on. The Derbys at Daytona were clocked at 85 mph coming off the banking. That is about 140Killometers per hour . My bike has the 75cc kit and the 22mm carb installed with no restrictor in the Exhaust. My bike runs about 85 MPH also. I weigh only 150 lbs and that helps too. The handling and Braking is great. I recommend the Derby.