If the fuse is OK then next thing to check would be the bulb (substitute with a new one, even if it appears the filament is intact). If this does not cure it, unscrew one of the door pillar switches. With a test lamp (two lengths of wire and a 12V bulb would do) connect one end to earth then the other to each of the wires which run to the pillar switch. One of these wires should be live ie the bulb will light. If not then try the same test at the fuse. This would let you know if there is a wiring fault at the fusebox or in the wiring to the switch. If the pillar switch does have a live feed and the bulb in the interior lamp is OK but not illuminating then the fault must lie either in a connection at the light unit itself or in the wiring going back from the light meaning your alarm fitters are the likely cause.

Sorry for such a long reply but hope it helps.

make sure the door sensors are tight, they earth it or summit, they dont work if its not in tight

My inside light was working right up till I had an alarm fittted. The fitters told me it was like it before they started playing about. Seems a little suspect to me as they have run some wiring down the same side of the windscreen bit as the lights wiring. Where does it go to? Is it hard to get to?

I have checked the fuse.