(unplug mains + if you are happy enough to look inside)
have a look at the bottom rear of the machine, can usually take a side panel off fairly easily (not back panel!).
look at the float chamber and micro-switch area.
pump could be faulty, float could be jammed (dust mush) pushing against the micro-switch, micro-switch could be faulty and jammed on, or it could be a control board fault (more difficult to trace & fix).
If you are confident and know what you're doing, have a look.
If not, get a professional to have a look.
Good luck.
How it works - the condenser unit removes the water from the hot wet air, this fills up a sump below the condenser housing, this is then channelled to the lift pump + float chamber, the pump pushes the water up to the top water tank for you to empty using a pipe often seen at least partially on the rear outside of the machine, when tank is full the 'increased pressure?' on the pump (as the tank is full and its trying to push water into a full tank of water, not
empty air) means the water level in the pump + float chamber rises, pushing up the foam float and tripping the micro-switch (switch with a long metal arm where the float rises to meet, 2 wires connected to the switch).
Mark Hunkin, January 2009