Hi Chris and Paul,
Thanks for your replies.
The way I put it together was with the fat pan in first, then the diffuser over the burner and lastly the griddle.
The griddle is a really poor fit unless im doing something wrong, it moves around quite a bit which I can tell is going to be a real pain when cooking.
The three legs which stick out of the side which I think are the parts it should sit on are only just on the lip of the fat pan and so allowing it to move and rotate easily