Just put some music on my new S2 but when I hit the paly button it turns to a pause button, then pause back to play when hit again and I can't stop a song say half way through, it remains on pause.

Where's the stop button?

Up to now I've been using Task Manager to stop it but there must be a stop function? Not just pause and play?

Try asking the question on Google. Google knows everything!

There are loads of decent music players in the Play Store. I use Player Pro, has a sleep timer and very nice interface.

I don't use the stock music app on any of my devices.