This is because there is some debris (probably a coin) wedged in the inlet to the filter. It takes 5 minutes max to fix. Remove the pipe / hose from the rear of the machine and lie it on its back. You will then see a black flexible hose going into the filter, with a spring clip holding it on. Remove this and use your finger or long nose pliers to dislodge the debris and the cap will then turn. You can then clean as normal.
Note that even if you get an engineer out under warranty you'll still be charged if they have to do this to free the filter cap, as it is not technically a mechanical fault or parts fault
BRTurner, October 2010
