Thanks everyone for all the replies. We've already removed and burned
all the wood from the room - the problems set in we think when the roof
leaked and was replaced - but it was apparently wet for a very long
time (we weren't the owners then). As far as the dry rot is concerned,
there were no fruiting bodies, and no sign of any red spores. However,
the skirts had "cubed" and there was mycelium on the plaster, leading
up to the joists (hidden behind tiles). The main affected areas were
around the shower cubicle. We've gutted the room and will be refitting
it very soon. I do plan to coat all blockwork and all new wood in the
appropriate chemicals, and probably use plasterboard on the walls.

We're just trying to decide now whether to splice in new joists or to
just rip out the two affected (and one on either side of these) - as
well as the 2x4 that they're resting on, oh and the fascia board as
well.

Phil