Electrolux ESL6115 – Doesn’t Stop Trying to Drain

Here’s some context. The dishwasher is about 15 years old now and has worked quite satisfactorily up until a couple of weeks ago. The dishes came out grimy and I soon discovered that the water wasn’t heating up. After a bit of YouTube training, I dismantled it and removed the flow-through water heater and tested it. There was no continuity so I ordered a new one. That arrived and was fitted a couple of days ago. I set the machine to a 70-degree wash and turned it on. It went through the hot pre-wash and I could feel it was warm. This was immediately followed by a modest air-punch. That, however, was a bit premature. After the hot pre-wash the machine should drain, refill and the main wash begin.
From the sound coming from the plughole it seemed to have drained normally, but then the drain pump didn’t stop and kept on going for quite some time. It then stopped and the machine gave a repetitive double beep. The manual tells me this is something to do with a drainage problem and that I should check the under-sink spigot and the drain hose. Well they’ve both been dismantled and cleaned and there’s definitely no obstruction, but the problem recurs.
I’ve ran the drain hose into a bowl and started it again to observe the rate of drainage. It is quite quick and efficient, draining all the pre-wash water in a few seconds. But the pump doesn’t stop and continues to run. I took the drain pump off and had a good look at it.
The drain pump is quite clean inside with no obvious obstruction. The little flap that seems to act as a non-return valve immediately downstream of the pump is also gunge free and can do its job. In fact, judging by the way it clears the sump the whole drainage thing appears to be doing its job. It just doesn’t seem to know when its job is done and allow the machine to move on to the next phase. It seems very odd that this fault occurred immediately after I’d replaced the heater. I can only imagine that I must have disturbed something when I was going about it. I actually had the machine upside down when I replaced the heater; could that have done something?
I wanted to ask the brains on this forum if there is something I should be looking at? Is there some kind of sensor that I should look at/wiggle about/replace? If there’s anything else you might need to know, just ask me. I’d be extremely grateful.