Clog or AAV issue?

[Swifty_52]

I’ve had an issue in our main bathroom for the last few weeks or so and I’m pretty sure I’ve narrowed it down to either an issue with a clog somewhere down the line or the AAV valve.
Now I’ve plunged and plunged and plunged repeatedly the toilet, the sink and the bath, but the problem still remains. The issue is when the toilet is flushed, the bath especially and the sink gurgles and then if I was to fill the bath up then release the water, the toilet gurgles and the water level of the toilet fills up.
I’ve checked the external man holes and all is good, so I think I’ve pretty much narrowed it down to either a deep clog somewhere or the AAV. Any other ideas of how I can be sure what the issue is? I’ve removed some sort of polystyrene cover that sat over the valve at the top and I’m pretty sure the current one is glued on. I see some of the valves if in need to replace it can just be lines up and pushed in, so I was hoping I may be able to remove the existing and push a new one in. The soil pipe is internal and awkwardly behind the tiles boxing in on my bathroom. Any help or advice much appreciated

Cheers

 

[Swifty_52]