Nice pawn sacrifice with Nc6.

21 ...Rxb2! is the move that jumps out at me, and leads to a Q v. R scenario which is essentially winning, though your own 21... Bxa3! gives a sizeable advantage as well.  Either way, white is in serious difficulties.

One move later, however, 22... Rxb2! looks like the only clear winning move.

after 21. Rxb2, couldn't white respond with Rxd6?

calum: 22: Rxd6 Rxg8++ 23: Kh7 Qh8 #

Great Calum!  Congrats!  :-)

Great job! Yeah, that 26. Rd3 move was really irritating. Instead of trying to get the queens off at move 24 perhaps 24. Qf2 is an idea, with ideas of swinging across to c2 maybe. The bishop would be free to relocate as well. i think Gambitknight is right about the checkmate after Rxd6 in the 21...Rxb2 line but what just Qxb2 in return. The bishop e5 skewer isn't working because of Rd4 so Black just invested a  rook to win the exchange. So I think your 21...Bxa3 was actually best. I agree that 22...Rxb2 looks extremely strong. I could easily have missed something so you'l really want to put an engine to work because there's a lot of tactical possibilities there.

Looks like you've got some good technique going on in the rook ending as well. Nice.

21. ... Rxb2 22. Rxd6 Rb1++ 23. Ka2 Qa1#

Hows this line?

Well, Rybka is giving Be5 as the strongest move after 21. Qb5 with an evaluation of about 8 pawns in Blacks favour. Your move of 21...Bxa3 gives you a +5 evaluation. 21...Rxb2 is no good and Rxd6 is a big blunder and gets White mated. Black is only slightly better after 22. Qxb2  Be5  23. Rd4. (see, I was right!)