Thanks for the help.. The extruder in your pic there is the same as the one I bought.
The brass gear though is different to the one that came with my printer.
I have three here.. I've just put back in the one that came with the printer (the silver coloured one - I cleaned it up of course)
Could the state of the original gear hold the clue to what's happening?
I've tried two different gears and the slightly bigger/sharper one clicks more often. The original one waits a 30 seconds or so and then clicks.
Attachment 9102