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Probably more one for the DIY thread but if you are going to line externally facing walls in single skin brick wall stuff (aka victorian / Edwardian) you need to be careful about both vapour in the property being able to get out, and vapour coming through the brick and plasterwork being able to get out. That means either breathable materials, or very clever and thorough moisture barrier stuff.
Howard
I'm not an engineer, but from experience I have reservations regarding plastic (or Tyvek) next to rock wool. I've seen massive blooms of fungus in rock wool that was covered with plastic. The plastic meant the humidity couldn't escape properly. Re-did the wall with the same amount of rock wool etc just without the plastic and that problem did not re-occur.
For that reason I am sceptical to the suggestion further up thread where someone wanted to add a layer of boards on top of the insulation in their attic. That insulation then becomes a sponge for all the steam rising from your shower and kitchen.
That being said, I am sure having vapour control layers can work wonderfullly, if we're talking new builds where the engineers know exactly what they are doing from the start.