In reply to captain paranoia:
Having dismantled the old pump, it may well be that it just needed a good clean, to remove black oxide deposits (copper or iron; not sure). Still a bit rough turning, but, considering there's a date code 11/91 on the starter capacitor, it's been in service for about 28 years, it's not doing badly. Interesting construction and drive; I may have to look up how the motor works.
Just a heads up for anyone with the same problem; four allen-headed engineering bolts allow the pump to be dismantled (and the orientation of the electrics changed, to answer the original question...). The rotor pulls out of the stator assembly. Bit of manual and brush attention, and the seized rotor can be cleared. Save yourself £100-£135, perhaps. Or at least, buy yourself some time before it needs replacing.
May mothball for 28 years, assuming the replacement lasts the same time...