Do you mean you have mould and damp on a plastered wall??
Remember that foiled back plasterboards can not be dabbed to brickwork, you would need to put batons on the wall then screw them on to that, I have used foil backed plasterboards for this reason before and it has stopped damp coming through to the finished surface!
Yes its a plastered wall, the cause almost certainly is the age-old problem of being a north facing wall in a cold room, the wall also being the furthest away and its mildew mostly not actual damp..(clothes also mildew).do i need to put anything behind the battens and would they be half inch battens....also is this a case where a dehumidifier would help....and of course any other suggestions to cure this will be very welcome
If its a wall with no windows then if it were my house I would knock all the plaster off first and see what its like underneath...if its just very cold and not really damp then I would screw battens 2x1 would do it (and you might have to pack them depending on how straight the brickwork is) to the wall then screw the foil backs to it and replace the skirting board, you could do that yourself but you would need a plasterer to skim the wall if you want a nice finish to paint. (£100 max)
As for putting anything behind the battens, if its not damp then you wouldnt need anything and if it is then you could cut strips of tarpaulin behind it, you could also insulate it if you wanted to.
I'm not a builder or a joiner I'm a plasterer but thats what I would do as a cheaper alternative of getting a pro in.