Just wanted to post a thank you to this forum and in particular anyone who posted on this thread. I found it through google as my Bosch dishwasher had this problem, I read everything and got busy last night...
Firstly, I checked the waste pipes for blockages (I found the flexi hose from the dishwasher was transparent enough for a torch to shine through it) and removed the U bend at the sink end and cleaned it out. Normal sludge, nothing major.
Then I bailed out the water (a day or two old with bits in... yuk!) removed the food guard at the bottom of the dishwasher, and located the pump cover. One posidrive screwdriver bit later had it removed. Pump imepllor fine although a peppercorn had got stuck in that area.
Finally tried the non return valve, that little rubber flap. Came out with finger tips not pliers (strong nails of mine) and as the backwash entered the pump area, I found the culprit... a little white plastic screw cover about the same diameter as the valve flap had been stuck behind it! Must have been from my white kitchen units, dropped down the sink, got washed into the dishwasher soil pipe somehow and then worked its way down to the lowest point... the non-return valve.
Cycled the machine and it worked perfectly. Nice and shiny floor to it again, rather than waterlogged.
Thanks again, everyone.