If the 3 amp fuse is OK then it it almost certain that the pump is faulty. The pump is beneath the toilet roll holder. You will need to undo a screw to remove the pump and then cut the electrical wire and remove the water hose. Strip back the wire and connect the pump directly across the battery to check operation.
If you purchase a new pump it comes complete with electrical connections and heat-shrink sleeving so that you can connect the new pump and fit the sleeving to seal from the water. Make sure you slide the sleeving onto the wire before making the electrical connection.
Alternatively the flush mechanism may be faulty and this is situated on the roof of the cassette chamber. To remove you remove the cap from the centre of the flush knob and with a long crosshead screwdriver undo the screw at the bottom of the cavity. The mechanism is also held by four screws in the roof of the cassette compartment and a small mirror is useful in locating these. The mechanism will drop down and can be removed by unplugging the electrical connector. Check that the mechanism is not broken in any way, particularly where the flush knob shaft connects, and check that the two switches operate correctly. One of the switches is 'made' by compressing the flush knob and the second switch is made when you insert the cassette to ensure that you cannot flush the toilet when the cassette has been removed. I hope this helps