Don't get those wretched Dirt Monkey things from
Halfords. They're awful. I got some with my wife's Subway as part of the deal, and binned them after a couple of weeks. I replaced them with SKS Chromoplastic guards, which are the bee's knees. I've fitted several sets of these over the years (along with a few other types) and found these to be by far the best.
Make sure you order the right size, as they come in different widths.
http://www.chainreactioncycles.com/Models.aspx?ModelID=25793