Having diagnosed the reason(thanks Andy)for the siezure-oil pump seal slipping on it's shaft-,I am going to attempt to rebuild the motor myself.I have only ever done basic maintenance to bikes up till now,so this is a bit of a challenge.Anyway,I have sourced the parts and have got as far as removing all ancillaries,and am about to dismantle the motor.I will post photos of the damaged bits for comment,and may need to call for help as I ,hopefully,progress.