Michael at Fuse Recumbents has apparently had one of these bottle dynamos fail before* and suggested turning the dynamo wheel that rests against the tyre a few times. Apparently the wheel should turn smoothly but what I was getting was this odd bumpy ness alternating with smooth bits.
Based on my discussion with Michael I think this means that the bearings are shot and the dynamo is a writeoff. For the moment I'm going to fall back on the helmet light whilst looking at replacing the lighting system with a hub dynamo in the left wheel.
*Given that I've had the dynamo for the better part of 4 years out of it isn't that much of a surprise really.