Zanussi ZJD1219 comes up with an E20 error?

I have a Zanussi ZJD1219 which comes up with an E20 error when I try to user the dryer. You can see water in the drum. If I drain the water manually the dryer works okay. I cleaned the filter, but this made no difference. I also removed the drain pump and checked the impeller rotates freely, which it does. I checked the drain pump motor coil and thermal cutout with a DVM and both of these look okay (178 Ohm and 0.3 Ohm respectively). Where should I look next? Is there a drain 'valve' or similar on the drum?