RubberBiker coughed up some electrons that declared:

AIUI the diamonding pattern in the timber is characteristic of wet rot
and, as you say, the only remedial action required should be to cut
out and replace the affected areas.

Yea! Then I won't worry if the odd millimeter or so of the base of the wall
plate is rotton to the far left then - it would be a right PITA to replace
that bit as it disappears behind various things.

Replacing the bit that disappears off the right side (over the end external
11" wall) will be bad enough, but I think I can wiggle/hack that out once
I've acrow-propped the roof up and relieved the load. There's only one
further rafter beyond the right side and it feels sound to my fingers,
though I can't actually see it.

I'll stick some DPM under the new section of wall plate for good measure.

So long as the source of dampness
has been stopped,

It has - probably 10+ years back.

ther should be no more problems. Was that an
enclosed roof void with no ventilation?

Sort of. It is open to the roof voids in the main house via a 2" gap over
the main wall plate on the inside wall, but not to the outside directly.
This area is going to be a shower room, so I knew I needed to take the
plasterboard off in order to insulate, vapour seal and re-panel with
aqua-panel or similar - otherwise I'd get a ton of moisture going up into
that space.

I'll be adding ventilators into the fascia too (no soffits in that part).

Is it worth sloshing some fungicide around too?

Thanks very much

Cheers

Tim

PS

I should be grateful - despite the house having had some abuse over the
years, this is the first bit of serious rot I've come across, apart from
the soffits - but they always go eventually. I'll be re-doing them in a
couple of years.