After the initial plumber's visit, the pump went again. He called out for free and did a bit of work, but advised that if it kept malfunctioning a new pump would probably be required. After a few weeks of the pump only working on and off, I bit the bullet and got a new one. €700 for the pump, plus around €140 for the labour. But at least I can use the washing machine and the tumble dryer again!