Thule do several tow-bar mounted carriers.
I need to carry three bikes, so we use a
9503 carrier. It's kinda mid-range stuff, it cost just over £100 but it has the gutters to support the bikes on their wheels rather than hanging them from the cross bars, and comes with light board and number plate holder, so you don't need to worry about obscuring your car's tail lights. I like the 9503 as unscrewing 2 clamps means that you can tilt the bikes so you can still get into the boot. It's not quite perfect, I still need to remove one pedal so I can get the estate door open but that's a small price to pay, as the flip-side means that the carrier is close to the tailgate. It's stable, quick and easy to fit, packs flat for storage and we've used it a dozen times in 5 months, without a problem.
Bikes of a wide variety of sizes fit into the channels, from my large MTB with 26x38 tyres to the roadie with 700x25c tyres, to a bike for a five year old. It's very easy to use, just needs a good tug to get it fitted to the ball, then the kids can even use it to attach their own bikes. We use it for everything from 250 mile staycation trips to 10 miles trips to the off road area. Can't rate it highly enough.
I also bought, for a friend, a
Thule 970 Express which fits onto the tow ball in seconds. It only carries 2 bikes and they aren't as secure as the carrier I have, but attaching them is a lot quicker. Oh, and bikes on that will obscure lights and number plates. The good bit, though, is that it's only £42, pretty good price for that. It also folds up very small for storage when not in use.
The most solid option I've seen is the
Thule 908/909. Again, it has channels to carry the bikes on their wheels, but it's nearly 3 times the price of the one I got, and I can't see that it's worth that (for what I need anyway) as the extra rigidity means extra weight, making it a pain for storage.
BTW, the links above are all to a shop called
Cooks Motor Spares. They are pretty close to me, based in Chesham, so I can pop in there but their prices and stock availability has proved itself astounding. The reasons I got the 970 for my friend are that Cooks had them in stock when no-one else did, and they were £10 cheaper than most other places. Very knowledgeable staff (two brothers run it, I think the old man owns it) and it's a very "old skool" car spares shop, good guys, also highly recommended.