Changes to Modifier Keys not working on external keyboards

I really really really need to change my capslock key to act as control. This works fine for the internal keyboard, but is not working at all for an external keyboard.

I've tried changing modifiers other than capslock - same story - no problem on internal keyboard, no effect on external keyboard.

I've tried with both the bluetooth keyboard (3 battery version, not the very latest one) and USB alu keyboard (the full size one with the number pad). Same story.

Help!

Thanks

Tom