Which category?

I'd post it under General Builder. You're right that it may need more than plastering, and a builder can deal with the cause and then either do the plaster or bring a plasterer in. Before replastering, a few things need checking: Is the wall still damp? Years of leaking can leave a solid wall wet for months, even after the gutters are fixed. Plastering over a wet wall means it'll blow again. It may need time to dry out first. The outside of the wall. Check for cracked render, failed pointing, or a blocked or cracked downpipe that's still letting water in. Salts in the wall. Water carries salts into the brickwork, and they keep pulling moisture in even once the leak is fixed. The old plaster usually needs stripping back to brick, sometimes with a salt-resistant render underneath. Type of house. If it's an older property with solid walls and lime plaster, it should be redone in lime, not modern gypsum. Gypsum on an old damp wall tends to fail. Timber. Check any timber lintels, joist ends or skirting grounds in that wall for rot. When you post the job, include the age of the house, photos of the damage, and photos of the outside wall. You'll get much more accurate replies and quotes that way.