miele novotronic premier 500 - check drain flashes?

I'm not surprised there was a problem - after some building works I discovered that the machine door wasn't closed and there was a load of brick dust inside, so I cleaned out what I could first.
When I try to do a wash I get the "check drain" light flashing. We have checked the drain and cleared away any blockages, we have tried sending water through the outlet pipe that goes straight into the drain and that is clear. My husband has had the front off the machine trying to find anything else that could be blocked, he's even taken out the pump and put a voltage across it to see if it spins (it does).
All back together again but still the same problem.
Any ideas, PLEASE? I've heard rumours of secret Miele reset codes...