I've worked out that the dodgy handling and steering when coming to a stop isn't my badly worn tyres... Rather the headstock bearings seem to be worn and very notchy. I've read that this was a problem with some of the early bikes, due lack of grease applied during factory assembly and I know that due to the oil in the frame the grease around the bearings gets a harder than normal time, but should they have failed at 8000 miles of road riding? I've done literally a day of trail riding in that time. Has anyone had them changed / replaced under warranty?