Thanks for the quick replies guys. It is a strange one. Further to your comments I've been reading up on the ability to do this to a DC motor and as far as I can tell without changing the polarity by switching POS neg wires or rewinding the coils it shouldn't be possible.
To refurb all I did was separate motor, free and sand the brushes, put the motor halves back together in the same alignment as previous! Unlike what appears to be a US version of the fan motors, the input wires are hidden on mine (ours?) by a black shroud so impossible to replace by mistake.
Any more ideas from anyone welcome!