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.