Hi Guys, this looks a trifle dangerous....what is the correct way to stop this happening?

Call someone who knows what they're doing

Probably NOT Grumpy, from what he's been saying

If you can't judge how a tree wants to fall, you shouldn't really be felling it.
The tree "sitting down" on the bar can often be prevented by using wedges as you cut (plastic ones, please - the steel ones are for splitting timber, and should only ever be hit with a wooden maul, not a sledgehammer)