Chess Puzzle

Welcome to the first chess puzzle of the season. Time to dust off your chess muscle after the summer break and get it exercising again. Played last night in the excellent County Championships (u125 section) in my game (as White) against Howard Philips (Black):

White to play.
It's my move, and I have a good attack on the go on the queenside against the Black king. I was looking closely at 19. Nc6 with a decisive fork on King and Queen, but this is prevented by the black Knight on e5. So... I thought, I would play 19. f4, kicking the knight away and then play 20. Nc6+ - I could see his reply 20 ... bxc6 could be met by 21. Qxc6! and mate is unavoidable on either a8 or b7.

Great! But then I realized there was a major flaw in my analysis... What is the flaw and what would you play instead?

Answer next week!

See you at the Annual Bash at Spinney Hill Working Mens Club on Thursday night - always a good night out.

Phil.