Witter Clamp-On Towball Mounted Bike Cycle Carrier from Ixplor 

The ZX304 bike carrier from Witter is platform based and takes up to four cycles of any size including a child’s. Clamping directly to the towball using a lockable clamp-down lever making it suitable for any type of towbar.
ixplorstore.com/gb-en/a/bike-racks