Thanks to “the kick inside” for your input.

I put my trust in the transmission workshop mechanic because he told me that he had worked at my local Volvo dealership in the past, and it was the Volvo dealership that recommended him to me in the first place. So I wonder why he expected to find a clogged internal filter in 850’s transmission if they are famous for not having one? Could it be different spec in the U.K.? I will look into this with Volvo.
Maybe the mechanic in question was sweeping the floor in his former occupation and I was suckered into trusting him with the old “Oh, yeh I used to work for Volvo” It was probably “Oh yeh, I used to work for Ford” with his next potential victim.

The vehicle is back home now, I am seriously considering doing a flush myself and see what effect that has before spending my hard earned cash on a replacement transmission.