Yesterday a representative for AEG came to examine the plastic fill pipe. His first words when faced with the brass coloured ring in place on the end of the supply pipe, and the rest of the plastic hanging loose, were "Oh. Yes." Two important sentences. He quickly confirmed that it was nothing to do with installation, but was clearly a faulty pipe. He went to his van and produced a normal plastic fill pipe without the detector on it. But I said that I do want to know why the original failed. He said that he had never met the problem before, but that he would ring up and see whether his colleagues had come across it. He did that and yes. there had been one or two cases, and there was a modified fill pipe that he would arrange to have fitted. I think he must have been told that he must only replace with original equipment and he seemed slightly less cheerful, so perhaps they told him to be careful in case we get ugly about our flooded kitchen. He did however note on the worksheet that the floor was damaged.
So it looks like a design fault rather than a manufacturing fault. I have been racking my brains to think why this problem may not be affecting thousands of customers, and have come up with a possibility. The width of the connecting ring is only 7 or 8 millimetres, whereas the amount of thread on the end of the supply pipe valve is slightly more, which means that when fully tightened the supply pipe has pushed against the rubber washer in the end of the fill pipe and eased the ring out by a millimetre or two. That may have been enough to sufficiently break down the friction grip which is all that holds the connecting ring in place. This could not happen with normal pipes because the connecting end is cup-shaped and can't screw onto the supply pipe more than a fixed amount.