Further to the earlier thread, I (that's probably the royal I in the
form of my Brother in Law) am going to open up the ext roof and take
the stack up beyond the eaves.

The present AAV works fine for negative pressure, but not for positive
pressure in the drains (That used to be dealt with by the neighbours
SVP, but it turned out his inspection chamber was full of rotting
meat! (anyone lost any relatives in the West Sussex area??!!)

Thankfully since clearing it out, the smell has now abated a little.