Thanks for the tip about "fifteensquared" - for that matter, I could use the "cheat" or "solution" option at the Guardian crossword website; however I don't like doing that!
I sometimes get the correct word but don't know why! The excellent blogs on fifteensquared give a full explanation and fill in any gaps in my general knowledge. I never go there until I've finished the crossword, or have got as far as I can with it.