I had a closer look at the magnets and can't see anything significant missing. There are some marks on the magnet edges but I suspect this happened as the broken pieces of plastic came off and no doubt spun around for a while before exiting. I will try and get some advice from an auto electrician on what could be done to rectify. Interestingly the FSM (disassembly-engine, alternator rotor pg 86) has 4 photos of the rotor, 2 don't appear to have any material on the magnet edges, 2 do - hmmm.
Meanwhile I have tried tackling the water pump. Despite lots of penetrating oil and lots of wiggling over several days, the impeller wasn't coming off the shaft (yes had removed the circlip). As the cylinder is off, I was able to undo bolt at far end of impeller shaft and take shaft out with impeller still attached - now soaking with penetrating oil from the other side as still can't get it to budge and may leave it stuck - the rubber seal and sealing seat in the impeller look fine. But what I saw past the mechanical seal retainer didn't look so good (first photo) so off that came. The space between retainer and oil seal in cylinder was full of gritty crud and excess gasket compound (second photo) and the drain hole well blocked (third photo). The FSM says this drain hole should not be blocked, "if it does, the coolant may mix with engine oil". Ok, this cavity has an oil seal on the inner side and a liquid gasket sealed retainer on the outer side - doesn't either or both have to fail for oil and coolant to mix? And how did all this crud get in there?? After clearing the drain hole thought I would see where it goes. It exits the cylinder block (forth photo) just above and outside the crankcases, in the front of the engine, directly in line to catch all the shit from the front wheel. I can only think the crud has come up the drain hole. Mental note; don't pressure wash front of the engine near the drain hole exit!
The mechanical seal looks ok and actuates nicely. However I would like to give it a good clean without taking it apart - should I use solvent, coolant, plain water or just a squirt with compressed air? Plenty of cleaning to do while waiting for a new TC17317 oil seal.