The leak is most likely going to be the cartridge being honest.Can you tell if the water leaking is Hot or Cold?
If the water is hot there is an o'ring in the rear of the housing/body that holds the cartridge.
Aqualisa can offer this o'ring free of charge,the part number is 147702.If however this o'ring doesn't stop the leak you will need to renew the cartridge as the internal seals would have failed and these cannot be replaced.
Hope that this info helps.
Mike, August 2010