I'm having to replace the RH Crank main seal (bike was consuming gearbox oil) so while I'm in there I thought about replacing the water pump seals as preventative maintenance.
In the end I decided 'if it's not broken, don't fix it'. I'll probably regret that, the bike's got 280 hours without any pump problems but I'm probably pushing a friendship at this stage.

I did take the impeller off to take a look and found that it has been rubbing on the housing. It's caused a small amount of galling on the back of the impeller, some flakes of plastic in under there and obvious rubbing wear on the housing.
My solution was to put the impeller in the lathe and take about 0.5mm out of the back recess. I checked periodically with bearing blue until I just had full clearance.
There was slight rubbing at the perimeter flange too but I solved that by rubbing the impeller on some wet & dry on a flat surface.
Previously I could hear the rubbing when I turned the impeller up to my ear. Now that's all gone. Pic after machining.

Hopefully this will avoid debris in the coolant, possible damage to the outside seal and potential failure of the impeller.

As an aside, just in case anyone comes looking:
The Primary Gear Bolt is RH thread with Loctite 243 on it. It took a really solid go at it with heat and a pneumatic impact wrench to get it off. I snapped one hex driver (not a top quality driver).
The seal, bush and O Ring all need replacing at the same time.
Pretty easy to remove the seal - pull the bush out with a couple of magnets then lever out the seal.

When we had a leak, the black plastic appeard to have been the cause. Time well spent!