Always been more of a d3 man myself
Me being spanked playing 1. c4

Hi jaydeeuk
I prefer playing 3.g3 and 4.Bg2 instead of 3.Nf3 because this controls the square d5 and hinders black from playing d5 early.
Anyhow you both played theory for quite a long time. I don´t like 13.a4 at all. It weakens your pawn structure unnecessarily. 13.Ncd5 looks much more naturally.
14.Ncd5 also looks more convincing to me than 14.Nb5 because Black won´t like to exchange the black squared Bishop at that stage.
Instead of 16. Rxf4 I would prefer 16.gxf4 in order to keep control over the square d5. The black Knight made very good use of d5 later on exploiting the second weak square g4
24.a5 is a desperate move losing a tempo, exposing the a-pawn and giving up control over b5. With 24.Bf4 you would have forced black to exchange that terrible Knight and you could maintain some drawing chances with a kind of fortress. After 24. .. Ne5 the position is quite desperate.

I went to http://chessok.com/?page_id=352
to check the opening moves. Amazingly after initial few moves both black and white are following the first option moves till 16. ... O-O (except for 12. ... Rc8, which is the second option).
Breaking away from theoretical lines is one of the important concepts in chess. The reasons are
1. To get into a position that you are comfortable
2. To get into a position where opponent maybe uncomfortable.
I regularly play d5 as opening against e4 which doesn't have deep opening. For me it looks like black played carefully to pull you into a position he is comfortable with. Maybe you could try to break away from "the best" to "best" where you are more comfortable. If your opponent had chosen 12. ... exf4, the position would have been entirely different.
Played a game recently, using my usual c4 opening, and for the first time in quite a while, I was completely outplayed (have played 3 games against this guy, destroyed me every time, I'm 1900 on here, him about 2000).
Where the hell did it go so badly wrong?! I don't think I played badly, but I was no match for this guy anywhere on the board. He said hes no specialist against the english. I know he used an openings database (I didn't mind at all, even though it put me at a disadvantage), but this one game has really put me off from using the english now.