Damp and mould: checklist
How to remove mould so it does not return: find the moisture source, test the ventilation, dry the space, strip affected materials and only then treat.

[ ] Find the moisture source instead of scrubbing the patch
    A leak, condensation, ventilation, a cold wall, drying laundry. Without this any treatment returns within weeks.
[ ] Test the ventilation draw
    A sheet of paper at the grille. If it does not hold, the ventilation is not working, and that is fixed first.
[ ] Check pipes, joints and the neighbours
    A hidden leak produces damp with no visible water. The riser, connections, bath joint, tile seams, the flat above.
[ ] Deal with condensation on windows and walls
    Cold bridging, the handle setting, heavy curtains, furniture against a cold wall. Condensation is moisture from the air.
[ ] Adopt short bursts of ventilation
    A few minutes of a fully open window beats a permanently ajar one: the air changes without chilling the walls.
[ ] Stop drying laundry in a closed room
    One load releases litres of water into the air. If there is nowhere else, only with a window open or extraction running.
[ ] Move furniture away from cold walls
    Five centimetres of gap gives circulation. Mould behind a wardrobe is almost always about that.
[ ] Dry the space out
    Ventilation, warmth, a dehumidifier. Treating a damp surface is pointless: the growth stays inside the material.
[ ] Remove affected porous materials
    Wallpaper, some plaster, silicone joints, soaked plasterboard. Wiping the surface is not the same as removing the growth.
[ ] Work in gloves and a mask, with ventilation
    Spores and products irritate the airways. Never mix products and follow the instructions for the one you use.
[ ] Treat the surface once it is dry
    As instructed and with the stated contact time. Drying first and treating second matters more than the choice of product.
[ ] Keep an eye on the humidity
    A hygrometer is cheap and shows whether your routine works. Without a number you are arguing with feelings.
[ ] Call in professionals if it keeps returning
    A large area, several rooms, a smell with no patches, allergies in the household. Not a job for a brush.
[ ] Establish a routine that stops moisture building
    Ventilation, extraction, drying wet surfaces, laundry outside the room. Cheaper than any repeat treatment.

Checklist from Unload: https://unloads.me/c/damp-and-mould