Monday’s position was taken from my game with Black against Joshua Pirgon in the recent match between Richmond A and Surbiton A.

I’d just played 20… Bd5, having seen 21. Bxd5 exd5 22. Rxd5? Nb4!, but then realised that White could switch the move order and play 21. Rxd5 exd5 22. Bxd5, which appeared to give him a winning advantage.

That’s what happened in the game, but we’d both missed that after 21. Rxd5? I could still play 21… Nb4!, winning the exchange. Stockfish tells me chances are still about equal as White will pick up at least the e4 pawn in compensation.

The best move is 21. Nb6!, when, after 21… Bxc4 22. Nxc4 White has what Stockfish considers a winning advantage due to Black’s weak pawns.

The only other move to result in a White advantage is the complex 21. g3!? Qf3 22. Rxd4! e3! 23. Rd2! exd2 24. Bxe6 Bxb2! 25. Rf1! Re8! 26. Bxf7+ Qxf7 26. Qxd2 when White is a pawn ahead, but Black has some practical chances.

If you found that line without using a computer I’d be very impressed!

As I only asked you for the next move, full marks for 21. Nb6, partial credit for 21. g3, but nothing, I’m afraid, for Rxd5 (or Bxd5).