Update:

I have given Walnut to all of them and for Gizmo 4 doses of Holly, Mimulus and Chicory a day. I know it can take longer than 3 weeks for the symptoms to improve, but in fact they are getting worse.
Gizmo is peeing every morning now between the litter boxes and is generally in a bad mood. Yesterday he had one of his meowing fits where he just walks around the flat "complaining" and I can just tell that he's not happy.

In fact while I was writing that, I heard one of my female cats use the litter box and then she started hissing which means that Gizmo was bullying her off the litter box. And now he's walking around the house meowing in a deep voice.

Do I stop the flower essence? Any more ideas?