Gurgling after flush, then loo drains too low and bubbles. (UK)

[TrevorCoultart]

Hi all, seeking advice on my toilet. (Layman here, please be gentle with me.)

When we flush the loo upstairs, it flushes fine (therefore assuming no suggestion of any actual blockage) but after the flush we get a rumbly gurgling from deep within the pipework somewhere and the water level in the pan goes too low, to the point that bubbles regularly come back up (and continue coming intermittently). I've tried buckets of water and detergents, and a couple of different 'plunging' techniques but nothing seems to change. What's my next approach? Was imagining trying something like a toilet snake/auger but very mixed reviews about success of these and I'm not sure if they'd be the right tool anyway.

Context: Three story house, this bathroom is on the top floor. Toilet drains down soil stack that passes all the way down one corner of the house, passing kitchen on middle deck and utility room (with a second WC in) on ground floor. No issues with ground floor toilet.

 

[oz-plumber]

You have a blockage somewhere.
It could also be a venting issue, but this would have been apparent from when the toilet was installed.

Best get someone in with a camera to determine where the blockage is.
Cameras can get through the toilet bowl without removing the toilet.

 

[Ben-gee]

As above , could be a venting issue if you have an air admittance valve (Durgo) which has now stuck.