This is a rack you attach to your roof bars. It takes one bike with wheels on and sits upright in a 'gutter' while an arm attaches to the down-tube. I already have one and was surprised at how sturdy it is, considering it's the cheapest of the cheap, now I've ordered another.
There are slightly cheaper options but the 'deluxe' comes with a locking knob on the arm to give you a fighting chance against the crims when you stop at a service station.
Rutland do free delivery over £20 so you can either pay delivery or add something to get it over the £20 mark like I did.