Thanks - I tried exactly the same procedure again and this time it worked. I think the problem last time was that a change of trusted networks has to be made on one of the Sonos devices, and the change is then propagated to the others on the network. Obviously this takes time (probably quite a long time if you have more than a few devices) and I suspect that I didn’t leave it long enough before disconnecting the Boost. The result is that only a proportion of the devices know about the “new” network setting, and those that don’t disappear from the system.
From an hour or so of testing, upgrading to a mesh network is definitely worthwhile on my system. For the first time since I installed it around 2 years ago, I can play FLAC files properly, without bandwidth issues when jumping between tracks.
It seems that the advice I was given last year to turn off the Boost was correct, and that it does become too much of a bandwidth limitation when the system grows beyond a certain size (though what that size is no doubt depends on a whole load of factors, including things that may change daily, such as the interactions/conflicts with other wifi networks in neighbouring properties).
If considering a mesh system, it probably makes sense to go for a tri-band system, as these have dropped in price a lot recently and are no longer “premium priced”. The theory behind this is that the Sonos devices are dual-band, so the mesh nodes can use the third band to communicate with one another without conflicting with the channels used by the Sonos units.