Zanussi ZSF6126 constantly draining

As subject. Last cycle was a rinse and hold, then put it on yesterday to do a normal wash and:

As normal the drain pump is the first thing to start up, normally runs for perhaps 5 seconds, then starts filling. This time the drain pump just continues for several minutes, then the machine stops showing fault code 2 (problem draining). However there is no water in the machine.

If I pour water in the bottom of the machine, it pumps it straight out, so the pump is OK, and there is no blockage. The anti siphon valve works too (don’t ask me how I know that )
I’ve taken the bottom off and tiddled with the anti flood switch which seems fine. There is no water leakage. The filter is pretty clean (was cleaned recently) and was correctly in place, so I don’t think anything should have got into the pump. It makes the normal noises – quiet when pumping water, a bit gurgly when scavenging the last bit of water.
This machine has no accessible siphon tubes etc. inside, so I guess it must have some kind of level sensor underneath but at a quick glance I can’t see it.

Can someone point me at what to look at next?

Thanks

Nick