Hi Smurff, the comments about watching your hitch weight above are correct, but taking those into account I still decided that the hitch mount was right for me - I got one off eBay in the end, I was in no hurry so I was able to let a few go when they reached a higher price than I was prepared to pay, and finally got one for about twenty quid.
Be aware that some designs make your trailer electrical sockets difficult to access if they're in the normal place - I'm going to fit the bike rack mounting bracket, and convert to 13 pin, at the same time, and I plan to mount the electrical socket above the mounting bracket, but think about that now before you decide which bike rack to buy.