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Just back from vet.
He emptied both sides and lots of fluid came out, with bits in it. Gave him a jab of colsone (or something like that) to reduce the very obvious inflammation, and a spray of doggie deodorant.
Vet said just like all your answers...sometimes it only needs doing once, sometimes regularly but probably that means about once a year, sometimes becomes a real problem and a more permanent solution needed.
Having seen the vet do it, I really doubt I could to it myself, hopefully I won’t need to! Couldn’t believe the amount of stuff that was squeezed out!!!
Vet also reckons diet makes no difference, some dogs are prone to it, some not. Our pup has a very strict diet and his poo is always very firm.
Thank you all for your answers, you certainly live and learn :)