simple job - remove split pin and take prop off. If it has never been off it might be stuck on the shaft but a bit of light tapping and levering will free it. undo 2 x 10mm bolts on gearbox, lift off cover plate and there it is.
If the impeller looks ok it could be blocked cooling channels in the head which is a slightly bigger job although not difficult.
I'd put money on the tell tale being blocked, try poking with some thin flexible wire. A very common fault on small o/bs. Otherwise as already said it's the cooling channels, head off and clean out. Easy but obviously new gasket needed.
Like other posters here, I suspect you will find it is the cooling channels in the cylinder head that are blocked. They are small and easily clog-up with salt/sand.
It is a fairly straightforward job to clean them - I did mine recently and (fortunately) was able to re-use the head gasket.
It is possible that there is a blockage in the cooling feed pipe which runs up inside the leg, but not very likely, IMO.
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Thanks to you all.
As you said simple jobs, about half an hour in all.
Impeller was ok, a little salt in the cylinder head waterways was all I found. Running ok now, think I got away without a
new head gasket. I guess this is all due to short runs and stowing in a locker instead of on the rail.