Leave them alone.

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Must I prove to you how good we are together?

I recommend you just try and move past this, don't let him get away with leading two girls along because he will think it's an OK thing to do, I suggest you tell him, it's her or you and then he has to decide. Don't be strung along just to get hurt, never let yourself get in a position in which you have a high chance of getting hurt.

If he cheats on her with you, I really wouldn't get back with him because it shows that he's willing to cheat on people and there is a chance he will do the same thing to you with someone else. Just, look out for yourself OK.

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we not doing anything together we just talking as friends mainly and he coming round as ive invited my friends round to hang out while my parents are away

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He can't have everything, he has to make a choice. He is ENGAGED.

Either way, you're setting yourself up so I hope you know that. Either you're going to be heartbroken or you'll have to deal with a lot of flack. I really don't think you're being honest with yourself because in your posts, you've said "we just talking as friends MAINLY," "we GOTTA behave like friends as he doesn't want to cheat on his gf," and "he wants to be with me." You are doing the wrong thing if you continue this. If you still do it, just enjoy this time of suspense because it's the greatest calm that will be for a while.