miele g646 sc plus F05 fault code?

HI
My dishwasher fills, overfills infact, but doesn't progress. instead of filling in one go in about a minute as it used to it now fills in 5 second bursts, continuing until it overfills and either stops or overflows in to the tray below and sets off the flood protection switch.
I went in to the service mode and it the light came on at 6 o'clock which is F5 - Drainage fault -heater pressure switch. I've tried spinning the impeller with a fork, I think it's moving but can't really tell.
Any thoughts/help would be very appreciated?