Shower plug hole bubbling up when toilet is flushed

[cockroach]

Only started two days ago. When both upstairs loo's are flushed the corner shower in the ensuite gurgles then bubbles up some water. Anyone have any ideas why. Thx in advance.

[buildersmate]

Yes. There's a partial blockage between the pipe where the waste from the loo goes down into your drainage system and out to the sewer. This is increasing the pressure of the air naturally in the upper part of the system to be above normal and its bubbling through your shower waste trap. There's only a partial blockage as the soil must be gradually emptying away - its just taking longer.
This can either occur over a period of time with a build-up of sludge or (more likely in your case, given your description) something has got stuck down there (disposable nappy??)
Probably needs rodding, but you could try drain cleaner as a first step. Be careful - this stuff is concentrated suphuric acid.

[cockroach]

thank you...will try mr muscle type stuff 1st then.