A P.S. to the above:
I‘ve just read a really interesting article on anal glands (as the devil would have it, Humphrey‘s been scooting this morning) It seems problems can also be caused by allergies, including intolerance to grains and certain meats - the list of meats containing possible allergens includes all of the standard ones, chicken, beef, lamb, pork etc, so maybe that‘s also something to look for......
The article is unfortunately in German so I haven‘t posted the link but if anyone‘s interested, I can.