Follow-up for anyone reading – frustratingly, the next day the thing stopped working entirely – showing an E01 code. There was no blockage, so likely a permanent fault. No time to get the control unit delivered before Christmas, (the one time we really need it!) so I bought a new machine rather than wait and risk it still being something else. Not very happy about it packing up after only 2.5 years. The lesson is not to ignore little failures/quirks, but to get them seen to inside the warranty period if at all possible.