So I got my Damp test meter (thanks aaronjb!) - Been testing various sections and sadly it does look like the tiles are leaking, they were put up by the previous owners around 5 years go.
So it looks like they need to come off and be re-done, there is a plumbing shop local to me, so I asked for a recommendation for a plumber. Someone came around today, for a quote, quite staggered by what he said!
Now its a small bathroom, just room enough for a tub, bog & basin. The main side wall which is tilled is maybe around 8 m2, the floor is also tiled (which he said he could just re-tile over

)
I asked for a price just to do the work, I would supply boxes of tiles. Just wanted the old ones removed, old plaster stripped off. Allowed to dry, re-plaster & re-tile. No problem he said, £3,000 should do it

I'm sorry how much? I'm supplying everything, and you want £3k
Fek me I could buy a whole new bathroom for that!
Hummm, will keep looking around for quotes I think!