Just taken the tensioner off and stripped it down, and surely enough if was bone dry and very stiff. There was also a small bit of metal under the seals (can just see it at the bottom of the photo): Not sure where it came from, as bearing looked intact. Seals were a little worn, but after a load of grease was packed into the bearing they fitted back ok. Just put it back on and........... ....... she's running like a dream again! The benefits of this place are already apparent!