Simpson EZI set 605 top load washer doesn't drain?

Simpson washer has been working perfectly until today and It's a second hand one. I put soaking wet clothes in it and set it on drain & spin and once it was ready to drain it made this weird screeching noise and beeped and PU was coming up on the display. I drained it by unhooking the hose from the bottom of the washer and then tried again on rinse & spin and it worked with two loads of soaking wet mats, but when I tried the third load with soaking wet Doona cover it filled the washer with water but then made the same noise and signalling PU. So now I have stopped it turned it off and emptied the washer. I am thinking the pipes have got something in them to block it, but not sure how to get to the right pipes to clean them out?