Copied from a private message reply by Cartesia, who hopefully won't mind me posting it here to provide advice to everyone else:
Thanks for your question. Sorry i've taken a few days to reply.
There is no point trying to access the switch from the inside of the dishwasher....you have to...
"Sure enough, took the bottom hatch off and located the pressure switch assembly on the side of the sump. It twists off once you release a retaining tab."
Can you tell me what you mean by 'bottom hatch'? I'm guessing you can't access the pressure switch through the sump with the filter...
I'm afraid the cleaning tip didn't fix it. In fact I'd only recently cleaned the filter so it was fairly OK. I wondered if the pump wasn't working so I poured in some water and ran a cycle and the water has drained away. So, if everything's clear and it's still showing a 'drainage problem' could...