£18 me old mate is a special price - it's got a £28 price tag on it. I'll buy one next time i'm passing if you like, but we'll have to meet-up somewhere.
The aeriel is designed to fold - mine too has to fold down to fit into the garage. Mounting will require a solid peice of stainless about 30mm x 50mm (guessing here) and 4 stainless bolts and nuts (plus washers?). I don't thing you get any with it. It can be adjusted for any angle.
I had some stainless fabricated for a custom fit, you have nav. lights fitted in the ideal location for the mounting base, i would say mounting on top of the A-frame would introduce too much stress. Don't know what to suggest really! There looks like two other suitable 'plates' bracing between the tubes, but this might be a little too high up also.