How to Fix Kenwood Chef Classic? Mixer works fine but liquidiser slow?

My Kenwood Chef Classic mixer runs fine (it's the model that runs the motor for the liquidiser on top at the same time as the mixer) however the motor for the liquidiser now runs slow ie the blades rotate slowly- it always used to run at the same speed as mixer. Does anyone know why this may be? It happened after a vigorous bout of liquidising. The blades rotate but too slow to liquidise anything, strange as it used to run at the same speed as the mixer so I thought they were run from one motor drive. Could it be a fan belt or burnt out part?
Thank you for any advice.