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I had this and as well as a blockage in the pipe had flooded the bottom causing the float thingymebob to activate .I scooped out all the water by hand,pulled it out,tipped it on its back,this drains all water out of it,(if you do this you will have water all over the floor so have some old towels handy),I then unscrewed the drainage pipe where it met the sink and shoved an old shower hose down it(it was the one off the shower mixer but dont tell dh),this dislodged the blockage,put it all back together and touch wood its working perfectly again.
I think the outlet must be blocked as have removed the filter and the area below is full of sludgy water. I've tried a plunger and a metal wire thingy but can't see well enough to direct them properly into the outlet. Any suggestions ?