Quote ="Robbo"Think I might go for it then
, I'm 5 ft 8 so I'm thinking an 18 inch frame?'"
[urlhttp://www.livestrong.com/article/187060-how-to-determine-the-proper-bicycle-frame-size-for-a-persons-height/[/url
Sounds complicated but its not really, but still the best way is to go and sit on one - remember you're buying what is in effect a road bike and not a mountain bike, a bigger frame on a road bike will make for a more comfortable riding position and not knacker your back in completely especially in the hybrid style where you don't really want to be leaning forward too far.
Personally I'd always go for the biggest frame I could stand astride over, adjusting the seat height so that your legs extend properly when cycling is nothing really to do with frame size unless your on a stupidly small frame, but being comfortable on the bike is down to the height of the frame and the reach to the handlebars and unfortunately you're always going to be taking a chance on that if you just buy online, I'd be tempted to go into a store that has a good range of hybrid bikes and sit on a few different sizes - avoid mountain bike sizing as you'll just confuse the issue.