Bubbling shower when toilet flushes

[Mattlehat]

Hi
As the heading says, when I flush the WC in the ensuite, water bubbles up through the plug in the ensuite shower. The loo is also quite slow to drain away.

House is 10 years old on ground and 1st floor.
Any ideas how to sort this out?
Many thanks
M

 

[Dan Robinson]

Blocked drain, faulty AAV, blocked stack vent.

 

[HERTS P&D]

Check outside manhole.

Andy