There are a number of possibilities:
Firstly, check that the wiper relay (located under the panel in the footwell) is fully inserted - they have been known to work loose.
Secondly, check that all the wiring connectors around there are sound.
Thirdly, it could be the motor itself, in which case a new one will be needed. They are about £85 and are reasonably easy to fit, if you have a basic set of tools and are reasonably confident at working on cars.
Cheers,
Mike