You have correctly identified E03 relates to the a drain fault, as explained here. Normally this fault occurs due the PCB timing out as its taking too long to drain the water in the sump.
Apart from checking the pump filter, have you filled the washer and watched it pump out, to check the pump isnt on its way out?
If its working 75% of the time on most programs its clearly pumping out the pre-wash and normal wash, and but failing on pumping out the rinse water, this could well be a PCB failure, if your confident the Aqua\pressure sensor, as the PCB will be given the input to confirm the water has gone, and to move to the spin cycle.
Hi Peter, I havent heard back from you, if you have everything you need, please rate the service received using the star rating, so I can close the case.
Should you need any further assistance please advise how I can assist you further.