I’ve literally been on this for 3 days, and it’s been driving me crazy, but i think i’ve nailed it down
Cat > Sub Cat > Tag Group
Restaurants > London > Chelsea,Wembley,Wimbledon etc
Cinemas > London > Chelsea,Wembley,Wimbledon etc
If a sub category has the same name and tag group as one from another category, topics don’t appear in one of the categories when you filter.