Thanks dmdilks!
Thing is, I think the powerstrip must tolerate a small amount of power drain on the switching socket, because it was working before. That is - had PC plugged into the switching socket, and when PC off, the strip would power down the other sockets no problem (with the PC PSU still on).
However, more recently, it's not doing this. Have tried unplugging all usbs - but still being over the threshold for switch off.
I'm not very good with all the power down states (S1,S2 etc), but have tried both states in the bios - made no difference.
A very frustrating problem.