Migratory salts can be quite difficult to deal with. Once the wall is dry, the salts may disappear, but the staining possibly won't.
Dealing with rising damp (which is much the same process in this case), usually involves removing all affected plaster and replacing it. Tony's idea (plasterboard & skim) is what I would do here. Much less messy, and pretty quick too.
Before you go that far, Ratts, try a couple of coats of something like "Zinsser Stain Blocker" when it's all dry.............
http://www.decoratingdirect.co.uk/viewprod/z/ZINCS/?gclid=COm8xIfX09ACFUHGGwod8qwGZQ
You may be lucky :o)