My opinion: If the player has lost possession, it is in 'open play', so there is no question of a tackle, and the LOTG regarding tackling does not apply. So you can attempt to gain possession from any position at any point after the player loses possession or control. However, if you make contact with the player to gain possession or you run to gain possession, either or both would be a separate offence.

Thanks Chris - I would agree - whats your opinion on how far that ball is away from the players thats lost possession before it becomes open play - 1 metre?

You can't be arbitrary as to a distance. As a ref, I play it as I see it.

Richard, I think you are correctly identifying that whilst behind your opponent you cannot tackle or attempt to tackle because that would be classed at least as a careless challenge and penalised by way of a free kick.

However I think you are complicating matters. The ball (whilst on the pitch) is in open play and I can therefore tackle or attempt to tackle my opponent in order to gain possession. I do not have to wait for my opponent to lose control before doing so. 

So if you are behind your opponent all you need to do is to move legitimately (walk) until you are no longer behind your opponent and then you can make your challenge to take possession of the ball. However you have to ensure at all times that you do not challenge carelessly, recklessly or with excessive force. 

Just because you change your position from behind to alongside or in front of your opponent it does not make it a tackle from behind. It’s not where you start but the point at which you actually make your challenge.

Thank you - as I thought and many thanks for your response  - just getting fed up with players moaning that I'm coming from behind when I'm actually reading the ball and moving my position from behind to side to front to take possession.  As a fairly newbie to the game I just wanted to clarify.  Thanks again