Finally got round to doing this today, in between rebuilding the end of the boom for the sailing yacht!
Was a bit of a pain to get at, but with carb, starter motor and electrical panel all removed, I could finally undo the nut one flat at a time (as there's no space to actually move the spanner), disconnect the gear linkage and drop the leg… (the same nut was also a right pain to get back on again - no room to manoeuvre).
The existing impeller seemed fine, but having taken it all apart and not knowing it's age, fitted a new one anyway.
Sadly whilst the engine starts and idles beautifully after the carb clean, still not pumping any water (despite being fine a few weeks ago!). I did blow through it with an airline while all apart, and all the passages seemed clear. Will have to work through everything again - given it's very low usage (has been owned by us from new), hopefully nothing serious!