Dot and dab the walls with 30mm insulated plaster board.This will cure it for ever.
Skirts sockets and switches off if there on said walls….unibond the walls first then dot and dab the boards,re-scim ,skirts back on,long screws for sockets and switches.
Boards are approx £20 per 8 x 4.
By the way.I’m a builder by trade and own a property development company so this is spot on advice…
It,s the external walls themselves that are cold spots.This causes condensation and mildew to form.
End of thread.. 😉