Hello third ear - thanks for the response. Fred36 is my better half and she's gone to bed proper poorly - so I'm answering your questions!
The existing paint - make unknown, but it's quite a glossy, shiny finish. Walls are flat plaster, but the existing finish is bumpy/uneven - due I think to many blobby, uneven coats over the years, by looks of it, so sand paper gets the nibs/raised brush marks but doesn't touch the lower parts.
Paint to go over is Dulux Rich Matt, (not Trade version) - white.
In our last house, we have previously painted matt over a vinyl silk, and had a "crazing effect" in a few places, which i'd like to avoid this time round.
Any tips gratefully received.