Stripped the lower leg off to reveal a knackered impeller
Obviously the fitment last time didn't go to plan
New impeller fitted (much better fit on the key this time)....and pumping like a full bladder
Note - impeller this time around came out of a genuine tohatsu sealed packet, whereas last time (different supplier) it came out of a plain packet with the part number stuck on it.
Ferrule of the genuine impeller was plastic; ferrule of the 'other' one was steel
Maybe one shop was selling non-genuine parts?
All good now though and thanks for the advice