For some reason the page seems to not be displaying all of the options at the top - ie. I've added Amiga games and Atari ST games, but they're not currently showing as 'drill-down' options.
I'm checking into why this is - I suspect someone's broken some feature, but in the meantime, if you select the system you'd like, on the left, THAT logic works fine.
My apologies! :(
Right! - Just about to list more stuff!... :)