Thanks Catlad - but I have an update!
So started at 5:30am today to do as much raking out and pointing before the rain was due to set in at noon. Along the way, one of the outside bricks was very loose as most of the mortar was shot, so it need coming out anyway to re-set. Low and behold, I don't have a cavity wall, I thought I did as I knew I had two layers of brick. It appears that the inner and outer brick are butted up against each other, and in the brick I removed, was attached to the inner brick with a bit of mortar.
... so, the rain comes, and I have done most of the inverted V (Gable end) that makes up the two rooms on the first floor where the damp is apparent (side note - one room was dot and dab, the other was render and plaster. When I bought it, it was back to brick - I chose to do one room with render/plaster because it was a bit ski-wiff, and I caouold neaten it up with the render, the other was OK, so I dot and dabbed). The damp "spots" are coming through in both rooms, it manisfests itself as round damp patches at the top of the wall where the adhesive was behind the plasterboard), in the other oo it's pretty random.
I am not sure whether the new rain has made these patches bigger or not, or whether the pointing has helped, but I have taken a picture now as it stands and can compare tomorrow. I *think* it's got worse after the new rain despite the pointing, which is mostly done, but time will tell.
I need to re-paoint the outside wall - any recommendations regarding the best external waterproof type paint or paint with some sealing properties - as it appears any future issues on the outside of the wall can bridge straight across?
I am also worried that any salts that have transpired on the inner wall will remain there - anyone know how long it takes for thes salts to develop - I've only just done the darn wall!