its what i do for a living mate. easy mate . just prepare well and think ever thing through be for cutting etc.

roughly be about £300-£350 depending on fixings and where you get the wood . wickes is cheap but crap wood in my opinion . shrinks and moves alot . not cut that well either . some where like a BuildBase is half decent . 4x2 is fine for the frame if supported well .

where abouts are you based mate ..... mite be able to sort you out

this was the basic frame before i put legs to support it


this was after


this was one in a school pond




after i clad it


basically if i was doing yours id make a frame that was slightly smaller than the wanted area, then space the rafters at 18" centres , then you lad the other way . if you cant get one piece of decking all the way across and have to join . then mirror the join as if you were doing a laminate floor , if you get my drift. just pm me if you get stuck