Dont buy cheap ones I bought a set and they bounce around like a jelly on a tread mill at anything over 10 mph, will deffenatly be buying a new set next year.
I have tried three different types of towing mirror in the past and finally found the best for stability and clear view which are the Milenco Grand Aero mirrors with the two top clamping screws. Also if you happen to change your towing vehicle in the future they should be adaptable which I have found the case so although they may not be the cheapest on the market you only have to buy them once which you save in the long term.
Yet another vote for the Millenco Grand Aeros, flat or Convex (which I can recommend), but like diesel steve says, - with the extra wide arms. I too have a Series 7 Bailey and I couldn't get a proper view all the way down both sides with the standard arms. Here is the link.