Thanks for that Guys,
some food for thought.
I should have added that these walls will also be getting covered with insulated plasterboard, which will then be plastered - either tape & joint or skim, not sure if that makes a difference.
Depending on your construction method air tight membrance, sealants and tapes under plasterboard with tape and filled joints may suit. The benefit here is the minimal amount of wet work.
Maybe this is what I should consider instead, I didn't know that air tight membranes could be used over blockwork...
The blockwork has to get a scud coat of sand cement & lime to provide a key for the scratch coat render. It is this scratch coat in conjunction with the tops skim coat of plaster that achieves the air tightness.
My builder has given me a quote of € 17 / m2 to carry out one coat only, approx 10-12mm.
Points about the extra drying time and the need to take the photos are also well made, thanks.
Boots