Getting Blue Opal’s water pump off was a bit harder than I expected; the last time I did it, I made sure to grease the threads so that things would move easily. Things did not move easily, and I had to resort to a hammer tapping the spanner to get the bolts to move. I suppose I don’t really want the bolt to move by itself, but it’d be nice to know that I’m not about to wreck something when trying to take the pump off too.
The existing pump is a Johnson 10-24509-01, which makes it after-market I think. From the manual for the engine, I’d already got the correct Yanmar part number, and searching the ‘net showed prices ranging from 140 GBP ex VAT to 700 Euro – wide range for a simple pump!
The replacement has been ordered, and once it’s installed, I’ll find the full rebuild kit for this old pump, and rebuild it.
Cover plate 
A better view of the signs of damage 
Cover plate view, in-situ 
Profile view 
Showing the orientation of installation 
Pulley view 
Recording the vane direction