Added to this, I've had to rebuld the wall under my bay window and replace the dampproof course there, its going to be plastered by a professional with me buying the materials and doing the dogbody'ing - he said that he cement the wall then plaster on top - would that be ok? (after reading the above)
As said using a premixed renovation plaster is ok. or if mixing his own plaster he must use 1 part ordinary portland cement to 3ish parts good quality washed loam free sharp sand with a salt inhibitor/water proofer.
Sulphate salts that are often present in masonry are washed into the plaster by dampness. If these are able to migrate into the plaster (which they can if the correct plaster mix is not used - the above type of plaster mix is designed not to allow this), even if this is a cement based render then sulphate attack can occur. Also the correct type of plaster is required to prevent hygroscopic salts such as chlorides and nitrates from migrating to the plaster surface as these will absorb moisture from the atmosphere surrounding them.
There is a science behind the plaster and the use of this is more important than many people realise.