I received a text message from Trevor this afternoon. It said, simply, "At last!".
Having heard nothing from Colin all day yesterday, I rang him this morning to find that he was ready, finally to fix our problem, and that he would ring me back to tell me when he could do it. Because of poor mobile phone connection, I also managed to fail to chat with Trevor! In due course, Grace and I set out for an hour's walk, but were stopped just yards from the boat by a call from Colin, saying he would arrive within fifteen minutes.
Indeed he did, and he fitted a new pump. We tested it all, tried to get the PRV to open, and it was silent. We assume it is fixed. We are wary enough now to give it all a good 24-hour testing, but we feel pretty confident that the problem is over.
NOW perhaps we can start a bit of cruising!
The photos on this post are of the sky over the marina yesterday - with no post-processing. The colours really were like that!