New belt filtted as per the service manual instructions. (G0500.1AAH belt which is black not grey like the original and has a yellow strip running along it - Kevlar?),
Bearings in the idler pulley also replaced. Thanks for all the advice!
Only thing is that after a short ride the belt has moved a few mm off the rear sprocket. Will try to knock it back it in with a rubber mallet and see if stays in place...