The job is long done but there were a few issues along the way which might be of interest..
The cut to size panels were stored indoors to stabilise but remained reasonably flat despite only being primed on one side.
Mindful of the concerns about fixing over emulsion, I screwed temporary scrap roof battens near top and bottom. As there was no sign of bowing/lifting when these were removed, I decided to forgo retention screws.
The rebated trim strip had to be made up in the work shop. Routing 4m lengths of Oak was deemed impractical! so I settled for 2m. Even then, hand feeding led to *burn* spots where the tool remained in contact with the work while changing hands.
Diagonal lap jointing and unexpected *dips* in plasterboard led to an awareness that minor imperfections are clearly visible when viewed along the length. Oh well! I never told her it would be perfect!
An unforeseen downside... she now wants the stair wall done:-( At 42deg. parallelogram probably one piece per board!