I posted a few weeks ago about my 3.5 year old (can't find the thread now).
He was whining and tantrumming a lot. Hitting me, answering back and being generally unruly.
Someone recommended '1-2-3 Magic' by Thomas Phelan and I have to report things have improved enormously!
I was a bit sceptical when I saw how 'American' it looked, but the advice offered is very sensible and easy to digest.
Just wanted to say 'thank you' to whoever it was - sorry I can't remember who