I have had a number of brews made over the summer that did not attentuate properly initially. After bottling they have a huge carbonation and a nasty acidic aftertaste. All fermented at a higher temperature than the "ideal" 20C or so. None, incidentally, were fermented using the Coopers supplied yeast, all using Muntons/Wilko/Wherry yeast (which is the same?).
I put this down to continued fermentation in bottles, causing excess CO2, which dissolves in water to form Carbonic Acid, causing the nasty acidic aftertaste. (Same problem with the Oceans, if you look on the BBC website. Difference being that even my home brewing ambitions fall short of that scale
).
I have spent many a happy moment since venting excess CO2 from the bottles - mainly 2L PET bottles - by undoing the lids to vent the pressure out. This is pretty tedious, as it has to be done quite frequently, even with squeezing the bottles so as to leave room for the CO2 to occupy in between vents.
The good news is that it seems to work, in that the strange tastes - nasty acidic aftertaste and even cidery foretaste seem to clear with repeated venting.
If you crown cap your bottles, I saw a solution which involved removing the caps partially, standing the bottles in water and re capping later. Don't remember the details, but they are on here somewhere.
Hope these observations and ideas are helpful. :pray: