Here's a bit of good news. We were resigned to going and buying a Morco on Friday to replace the leaky Paloma, which would have been a Bad Thing for at least three reasons- it would cost nigh on two hundred quid, all the fittings would be in the wrong place, and on my past experience, Morcos just aren't as good as the no-longer-available Paloma.
But someone else here has had a similar problem, and it was mended by Paul on Prince, so I dropped him a text and asked whether he thought it was worth taking a look - and low and behold, he fixed it! It wasn't the heat exchanger this time (as it had been on Andante); that might have been too much to overcome, but an inlet pipe at the top. So, we have a fully working Paloma, which with the summer coming is a luxury beyond measure and a real bonus.