how can i fix my faulty porta potti?

i have a pota potti PP C2R fitted in my caravan, on our first camping trip the unit would not flush, after checking i found that the fuse had gone, i replaced this but it still would not run, can you tell me if it is the pump or switch.
If it is the pump can you please tell where to locate the pump on the un it.

rgards Bob Farrimond