Update. Work has been progressing, the impact of the extended leaking - which actually turned out to be (mostly?) due to to a blocked drain pipe has been....extended
The bedroom floor has been repaired, doubling up the joists with coach bolts/dogs in between and added structural screws. The joist between the partition wall dividing landing and bedroom wall (shown below) has been doubled, end to end
With water penetrating down the middle of the house for more years than I care to think about, the beam above the landing (above which is the double skin wall to the rear of the right front "london" roof segment) turned out to be completely rotten. On further investigation, I was less concerned as there is a strain relief arch, so it is not supporting anything more than the bricks below the arch. On further further examination, the bearing point on the supporting wall was buggered to all hell through the mortar disappearing and bricks going to buggery. Any premature removal of the wrong part seemed to engender a risk that the strain relief arch would fail, with corresponding amusement.
Current plan of record is to
(pre launch) hack off plaster and surround moving acrows around to allow support to continue
(a) mortar in the last brick in the arch which was removable by hand. Given the size of the bottom mortar wedge, I actually cut a wedge of engineering brick to increase its structural integrity. Wait to go off. (complete)
(b) hack out a segment of the rotten beam in the masonry, leaving the bit that seemed to be supporting a particularly loose brick, brick up the hole in the supporting wall tight to the loose brick so it has nowhere to go. (complete)
(c) hack out the last of the rotten wood, make good the hole left as a result.
(d) cut the rotten beam square, allowing an additional segment to be added (2 x scaffold board sections screwed together is the perfect size), with a 40mm x 3mm mild steel angle on both bottom sides, cut into the (now good) masonry. Not structural, but sufficient strength for the purpose.