Update: i've now managed it. I had to get a torque screwdriver to remove the lid (after unplugging it and turning off the water just in case). The mechanism by which the 'open door' button pulls a cable to lift the door latch seems faulty, so just manually pulling the cable opened it up. In future, we can just put some string under the latch before closing the door, which we can use to lift the latch when we need to open the door. Not ideal but will do the job!
Chris
February 2016