i would level the wall out with undercoat pref s&c with it being a bathroom then d&d on the newly straight wallEvening,
What is the maximum depth you can D&D? I need to build an uneven wall out ~50mm; is this possible? I will be using moisture resistant board and tiling (bathroom).
Many thanks in advance.
GW

You can dab strips of plasterboard at the worst places and then dab over that, if you can afford the space IMO batten and board is the best way of truing up a bad wall quickly and accuratly with as near to perfect results as possible. Before you use moisture board be sure to fully investigate the tiling section, aquapanel may be more suitable for you.
Sorry is that 1890’s or a typo? Cavity walls weren’t invented until the 1920’s so if it’s that old it must be solid brick; it is quiet important you get this right!External cavity wall (red-brick, 1890's, ~62cm thick).
So has the room it’s in always been a bathroom? Do you have any similar vents elsewhere in the property?Here is the vent:
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