Stone work(Sandstone block) is getting painted with a base emulsion
coat,builder says any bad areas will be filled in when the paint has dried
with sand&cement mixed with an expoxy.

Will it adhere soundly to a painted emulsion surface?

As far as I know a sandstone cannot be successfully repaird ie I was told
by a mason some years ago that to get a successful flat repair job on
sandstone it has to be randomly drilled and plugged with short screwable
rods and then primed with expoxy and then sand&cement.

Either that or replace the affected damaged block which is not viable in
this case.

Just found this...
A common method of repairing delaminating and contour scaling masonry is to
'face up' the defective block with cement mortar. That is not advisable for
several reasons:The cement mortar does not replace the load-bearing material
that has been lost. Structural failure requires structural repairs, not
facing up in cement mortar.The different rate of expansion and contraction
between the cement mortar and the sandstone will eventually lead to the two
materials separating, a phenomenon referred to as 'bossing' The use of
adhesives is sometimes used to strengthen the bond but this often leads to
the cement mortar patch pulling part of the sandstone off instead. Water
gets trapped behind the cement mortar and accelerates the rate at which the
surrounding sandstone decays.If pieces of cement mortar break loose they
fall to the ground which causes a hazard to persons directly underneath the
building.

http://anguscalling.co.uk/devcontrol/advice_note_30.pdf

The message
from "George" contains these words:

Stone work(Sandstone block) is getting painted with a base emulsion
coat,builder says any bad areas will be filled in when the paint has dried
with sand&cement mixed with an expoxy.

Will it adhere soundly to a painted emulsion surface?

Most unlikely.

As far as I know a sandstone cannot be successfully repaird

Of course it can. Cut block out. Redress face. Fill in behind with
stone and lime mix. Good as new.

The message
from "George" contains these words:

Just found this...
A common method of repairing delaminating and contour scaling masonry is to
'face up' the defective block with cement mortar. That is not advisable for
several reasons:The cement mortar does not replace the load-bearing material
that has been lost. Structural failure requires structural repairs, not
facing up in cement mortar.The different rate of expansion and contraction
between the cement mortar and the sandstone will eventually lead to the two
materials separating, a phenomenon referred to as 'bossing' The use of
adhesives is sometimes used to strengthen the bond but this often leads to
the cement mortar patch pulling part of the sandstone off instead. Water
gets trapped behind the cement mortar and accelerates the rate at which the
surrounding sandstone decays.If pieces of cement mortar break loose they
fall to the ground which causes a hazard to persons directly underneath the
building.

http://anguscalling.co.uk/devcontrol/advice_note_30.pdf

True. But what you can do if all you're wanting to do is bring the face
level with the rest of the wall is to use remove all loose and crumbling
material and level with lime putty which has none of the problems
referred to and does not retain water behind it.