In reply to the sheep:
For one or two bikes adult you may be ok with a hatch-mounted ("dangle off the back") carrier. I've heard/read anecdotal evidence that heavier loads can put too much load on the hatch's hinges. Not worth the risk in my opinion. Unless the bikes are mounted very high you'll also need a light-board with a number plate, and this needs to be wired to the car electrics.
I bought a towbar-mounted own-brand 4 bike carrier from Halfords last year and took four bikes to Scotland and back on a Ford Focus with a towbar that I fitted myself. It looked rather precarious, and I kept expecting to look in the rear-view mirror to find it gone, but we had no problems.
Here in Manchester, £300 to £350 seems about average for fitting a tow bar (inc. parts and vat). towsure.com sell kits and will also do mobile fitting, although I haven't used them, Having done it once, I wouldn't recommend fitting a towbar yourself unless you particularly enjoy that kind of thing: I found the mechanical part fairly easy, but the electrics are tricky. I eventually had to get help from a mobile auto electrician, which wiped-out any savings I might have made.
Andy