If you get blk mould on the wall does that mean my damp course didnt work

Either that yes if the damp and mould is lower down the wall and low middle tier too, there could be a few reasons to this issue could be leaking pipe in wall, no damp course like in your case I hear them damp proof injections are no good to be honest, or the floor is touching the ground so to speak floors either have damp proof membrane or are suspended off the ground with air flow. If the mould is high to high middle tier of the wall it could be a small slow roof leak somewhere or potentially a gutter issue if there fall and over following for years same if the pointing is shot or bricks in general bad condition. Mould is quite tricky to solve especially without seeing it, you wanna contact somebody who specialises in damp and mould failing that a good general builder or roofer might be able to help unfortunately it’s a process of elimination sometimes if it isn’t obvious.

There are many reasons this could be but, it has been known that some cavity installations can cause a breach in the cavity causing moisture to travel. If your happy its not coming from the newly done outside wall, ground level breaching DPC or overflowing gutters ect. Then maybe rent a dehumidifier and see if its just old moisture trapped. Some pics may help.... Best regards John Howes.

black mould on walls is a internal problem, and can be sorted by better ventilation, its normally because of condesation in the room and it will go to a cold spot and black mould will form in that area

It sounds like a lack of ventilation black mould on surface is due to condensation sitting on the surface. Have your air bricks checked inside under floor and others outside they can be seen easily . Have you had cavity wall insulation done as they normally block them to do job only it causes loads of problems. Let me know how you get on please. Kind regards Aaron