Genuine Yam XT piston (595cc) , it happened on a very dark stretch of the M4 which I didnt wantto stop on so I chose to carry on on a light throttle to the next services (5 miles or so). Lesson leant, I'm never very patient with running stuff in, it was getting up to 120deg C on the motorway, then in Holland I just thought sod it I'm going to enjoy the ride rather than pussy around.