I had the same problem on my 1994 Swift Challenger 490 SE. After reading Bob's (thanks Bob) post I decided to have a go myself. Just a couple of things to add really.
As well as fuse No 6 (10 amp) on the main fuse panel I discovered a fuse (in line) located near the pump switch. Couldn't find any reference to this fuse in the handbook, guess we're just supposed to know it's there. It's located in the cassette compartment near the connectors to the pump switches. It's a black knob which just unscrews. My fuse had blown, 2.5A which I replaced with a 3.15A but unfortunately this didn't fix the problem. There are actually two micro switches that both have to be activated for the pump to work. One switch is activated (closed) when the cassette is installed, prevents the pump from working unless the cassette is in place, for obvious reasons. The second switch is used to activate the pump when pressing the flush. Either of these could be faulty. Getting at these switches isn't to bad once you know how. First thing to do is remove the flush button by removing the stopper from the top (prises off) and using a long philips screwdriver undo the screw in the centre. Now need to remove the housing containing the switches, do this by removing the four screws holding the housing in place, there is one screw at the back on the right that is a little unsighted and fiddly. Now remove by unplugging the two connectors. I then used a volt meter to check the two switches, fuse holder and complete circuit in this housing. I appeared to have a problem with the fuse holder but that disappeared after a bit of messing about. Checked I had power at the connectors which I did. Assembly is obviously just the reverse. That just left the pump itself, located under the toilet roll holder which just prises out. Removed the pump by cutting the wires as Bob suggested. Tested pump on battery but didn't work, gently knocked it as suggested still no joy, then used a screwdriver to rotate the propellers of the pump through the access hole in the base of the pump. B**ger me, it started working, (try this before cutting the wires and you won't need to do the next bit). Reconnected pump using terminal block and insulating tape, tested in situ and Bob's your uncle (actually I don't think he is).
Sorted!!!! Cost me nothing other than 3 hours of my time.