Thanks to all who responded. As is usually the case, my very brief description left a lot of room for 'interpretation'.
To clarify, the loo has one long external wall 9" brickwork, which I am dry lining with sw battens, Kinspan and 12.5mm plasterboard and skim.
The room is very narrow, so on the long internal partition (4") brick, I hoped I could get away with just a skim finish on the (wishy washy painted) brickwork.
The low ceiling, as I mentioned, was cast in concrete, spanning between the external wall and internal brick partition. There is no evidence of damp in either the ceiling or the internal partition. Due to height, again I plan to simply skim this, and use the patterns from the formwork to help as a key.
The property was built by the council in 1948, and the thin paint wash was put on in the last five years to quickly brighten the loo up before we bought the house. If I stripped the brickwork of paint then I would consider hanging my minimal handrinse basin against this wall without any plaster finish, but I dont think the brickwork is nice enough (very smooth, flush joints, little 'character').
This is the first time I'll have tried internal plastering, although I did a succesful job of a white renderer retaining wall in the garden (using a wet sponge to smooth off afterwards). Since it's the downstairs bog, and small area, I thought it would be a good place to learn my mistakes.
Thanks again for the useful comments, as always.
Regards,