An alternative, if you’re after a cheap and easy rack is one of these for £15
http://www.ebay.co.uk/itm/CAR-TOWBALL-MOUNTED-FOLDING-2-CYCLE-BIKE-CARRIER-TOW-BALL-TOWBAR-BICYCLE-RACK-/201295402318?pt=LH_DefaultDomain_3&hash=item2ede24154e
works very well, folds up neatly, can be fitted in 3 seconds. A liberal dashing of pipe insulation to protect the frame though. It’s very secure, surprising amount of clamping force from the scissor action.
The two stubby arms folder over flush, then the two arms fold together. To clamp the bottom wraps itself round the ball as you open the big arms out. Its hard to explain, but I saw a friend had one and it’s just perfect for taking one or two bikes short trips when I dont want to get the big atera 4x rack out.