When my guys (now gone) were little, they had a round of Giardia, which was hard to diagnose and treat (and they happily went on with the annoying diarrhea for quite a while). We finally got it, with some medication though. I'm afraid this was more than 20 years ago, so I can't remember what was used. I assume you've got the possibility of that bug in the U.K., as well. You might ask your vet about it.

Here is some info online:

Giardia cati in Cats

Giardia in Cats

~ Becky in Tucson