Just a quick followup to this.

Had my bike serviced today as well as new tyres and cush drive and the problem seems to have gone away. Dealer said the cush rubbers litteraly fell apart when they tried to remove them. Funny thing is they seemed ok up until I had the chain replaced at 10k miles (not that much sprocket movement) but seemed to deteriate rapidly after that.

Cost was about 23 quid for the rubbers.