How to repair a Thetford C200 CWE Cassette Toilet?

Problems with the flush on a Thetford C200 CWE cassette toilet. When we press the button you can hear the motor running, but no water comes out of the flush. We previously had problems with black bits coming out in the flushed water, but by changing the type of flush chemical solution we used we do seem to have cleared that problem. We have looked on the Thetford website but can find no details on how to check the pump on the C200. Hope someone can help