Install insulated plaster board on battens on plaster?

The condensation is increased due to inadequate ventilation! Battening out the walls will create a cavity, and this will in part help reduce mould growth. The thick the insulation boards the better the heat retention, so given you are likely to disrupt the room, you may as well do so more effectively. It’s also a good idea to cut / fit a few small low level air vent in plaster boards etc. Additionally if none, fit trickle vents in windows 🪟 (or set too openings on half-lock to aid ventilation. Hope this helps Happy New Year 🥳

Ventilation is key. You could try foam backed insulated plaster boards and foam fix these to the walls then skim over.

I don’t see any point or advantages of battening out these days, unless of course you are intending to leave an air gap behind the PIR board. Just put composite board straight to the wall. It contains vapour barrier anyway. I usually dot and dab it, and use mechanical fixings aswell to get a darn good bite to the wall. In these times when we are all focused on energy efficiency, and homes become more and more airtight, this of course creates a different range of problems. Unfortunately we do need ventilation. It’s my view that all new windows should have trickle vents as standard. PIV units are also an effective solution these days to keep fresh filtered air circulating around your home and keeping mould and condensation at bay