What is this stench in my bathroom?

[Purplehonesty]

For three days now my main bathroom has being smelling like rotten drains or dead things and it is getting worse.
I've cleaned everything, cleaned out u bends and checked drains but nothing like that smells.
We had a birds nest in the roof above the bathroom and I swear I heard cheeping in behind the bathroom wall a few days back. Could a dead bird be causing the smell? It's so bad that even with the window wide open the hall now stinks and I can't imagine a rotting sparrow would smell like that.
We don't have a rat or mouse problem and the house (a barn conversion) was only finished a year ago.
So what could it be and where do I start?

It could be a blockage, such as a birds nest or leaves, in the vent pipe. This is the pipe that is usually on the side of the house and sticks up above the gutter and the water drainage pipes join onto it. All the sewage gases normally go out of the top of it but if it is blocked they can't escape so they come back into the house.

When we had a drains smell it was because a cap which should have capped off the main sewer from our "outflow" was missing. We paid £170 for someone to put a camera down to check. I'd rule out the dead bird first though

(sorry no idea how that smells!)

Ooh vent pipe, good call. Will check that out.
Have put boiling water and bleach down the drains and it has reduced the smell a lot so I think I have ruled out dead thing.
Will check all the vent pipes tomorrow
Thank you