The existing paint on a room about to be decorated is flaky and is
simply pulled away when a strip of masking tape is adhered to the
surface and then removed. Is it safe to apply fresh emulsion paint on
top of this coat or should it all be stripped back to bare plaster and
and re-sealed/primed ? The walls are skimned plaster board.

Stripped down to a good layer for adhesion. Tom
Work at your leisure!

There is only one layer as it's a 9 year old house. Existing layer is
thin standard Magnolia .... below is only skimmed plasterboard.

Was thinking about removing worst areas and using a base coat of
Emulsion:Water:PVA (4:3:1) . Any suggestions ?

Was thinking about removing worst areas and using a base coat of
Emulsion:Water:PVA (4:3:1) . Any suggestions ?

Quit trying to be lazy and just remove all the existing paint. Do you
really want the parts that aren't peeling now to start peeling after
you have painted on top of them?