Thanks very much for your reply. It is my suspicion that it is something like that.
Do you have any idea whether this could be stopped - without removing the tiles and the false wall?
We recently (around early spring) had an en-suite fitted which includes a mixer shower that runs off our combi boiler. The shower is an overhead rain style shower with separate controls for the mixer unit. The pipework now sits behind a tiled false wall and over the past few weeks they (or...