My indoor cat Alfie has started suffering with blocked anal glands

Dry food he is more fussy, well both cats are. They were on Arden Grange for years until they changed the formula, Storm continued liking it but Alfie went off it. To be honest Alfie is quite fussy with biscuits anyway, but we discovered he really liked the Arden Grange salmon so have just started giving them both the AG chicken and salmon mixed together. I can't be doing with separate containers, although Storm has taken an instant dislike to the salmon!

I had read briefly about feeding food with psyliium husks in but in the Royal Canin foods only. I wasn't going to bother as I am a bit picky on what brands I use due to how ethical the brand is. Although when I paid a local pet store a visit yesterday I succumbed to a reduced bag of RC oral care. Gave Alfie a small amount last night and he wolfed it down!
Does anyone know of any other foods containing this psyliium husk (I did a google search but couldn't find anything), and what are your views on ethical cat foods? I guess at the end of the day my cats health is more important. Alfie has bouts of digestive issues with either loose stools or hard ones he can't get out! The vet gave me some stuff to put on his food daily, and suggested I took him vets every 6 weeks to get his anal glands emptied.
Thanks
Suzanne