How to remove this plastic pipe elbow
In the UK (where question poster is located), there are several distinct types of plastic push-fit connectors. You can get specialized tools to help undo each type.


John Guest Speedfit
Wavin Hep2O
Floplast Flo-Fit
etc


Yours are Hep2o 



and can be released with a Hepkey - there is a tool selection guide.



Hep2O





Speedfit




FloPlast