Thanks for your previous suggestion, Chris. My continuing concern is that the 'old paint' adheres to the underlying plaster with varying grip. Just when I think I've removed the areas that have weak grip, then sand down and wipe over with meths, some more curls and shows loose, so I remove that, too. It's not desirable to remove all the existing paint - if this can be avoided.
On another couple of areas in the house, I did as you had previously suggested - diluted emulsion onto new but dry, feathered skim-coat. Let dry a couple of days, then a first top coat. Problems of bubbling emerged in several places.
I'm trying to avoid having that again, by the possible use of this Zinsser product *under and over* the new skim-coat on my firebreast.