i think top tether is for when you have a hook on the parcel shelf (as does missus bmw 1series coupe) or in the boot floor.
its for strap that is used for extra security of holding seat in place
 
My seat leon mk1 appears to have the two point isofix bars (red bars) in the back seats can anyone confirm this is the isofix standard or will I need to replace with a britax isofix mounting kit? Many thanks