Diagnosis? Toilet slow but aav looks OK

[vice]

Hi. I've a ground floor toilet suddenly backing up badly. There's another toilet plus shower and sink on the floor above. It does drain eventually but the ground floor bath that connected onto it gurgles like crazy.
Odd thing, on the stack there's a stadium aav. Nicely built into the wooden cistern, welded on, no way to remove. I'll have to chop it up to even take it out.

If I lift the valve putting my fingers underneath, doesn't change a thing. Top toilet drains no probs.
Any diagnosis?

 

[Hugh Jaleak]

Still sounds like you've got a blockage downstream. Gurgling is air being pushed back through the point of least resistance, i.e. the bath. Check any manholes downstream, and if they're clear, get someone to flush WC and see how quickly the discharge arrives at the chamber.

 

[jeff the gasman]

Blocked sewer.