Bike rack when I've already got a roof rack.

Hi All,

I have a defender 110 with a roof rack more or less identical the one in this picture:

http://i525.photobucket.com/albums/cc332/LandRoverAnorak/Forum pictures/RoofRackfront.jpg

I want to safely transport 2 adult + 2 kids bikes in the easiest way. Can it be done just by strapping the bikes down or is there a magic attachment I can fix to the roof rack to carry the bikes in the standard upright position?

I have a spare tyre mounted on the rear door so I guess that's not an option.

Anyone solved this problem in a cool and cunning way?

Thanks

Tom P-C