Go up into the attic and take the AAV off the top of the stack, place a square of toilet paper over the top and get someone to flush the toilet and empty a few full basins at the same time. See what happens.
As suggested by both Hugh and Andy, one of the main reasons for the bad smell and black mould in the attic is due to a blockage in the outflow and there is moist sewer air being forced up and out the top of the soil pipe, that's positive pressure caused by the displacement of the air in the stack when water is being introduced.
If the drain is shared and it is if 4 houses use it, then it's the water authority's responsibility to sort out.
As suggested by both Hugh and Andy, one of the main reasons for the bad smell and black mould in the attic is due to a blockage in the outflow and there is moist sewer air being forced up and out the top of the soil pipe, that's positive pressure caused by the displacement of the air in the stack when water is being introduced.
Would I need special equipment to unblock the drain or can I use a hose?
If the drain is shared and it is if 4 houses use it, then it's the water authority's responsibility to sort out.