Andrew
We just had the repairman around (had to get it fixed or get a replacement) and it turns out that the fault in question is one of two things. One, it's the heater unit gone pear shaped. Second, you have allowed the water tank to overfill (just like my missus did) and the water drained into the bottom where it collected fluff. The pump activated, thinking it was time to pump more water up to the top, but it was jammed with watersoaked fluff.
So if you fancied some DIY repairwork, try that first!
Sam, October 2007