what is sodium metabisulphite? that is a rhetorical question. not to be a jerk, but it sounds like you already know the answer to your question!
i'd also address the quality of the beef. first off, it's 93% lean? that sounds like a dry burger. try 85%. you need some good fat content in a burger. second, i'm assuming that the beef is loaded with hormones and all kinds of no-nos... but i'm not sure, i don't know the source of it, just the ingredients.
do yourself a favor. go to the butcher shop and get some ground chuck, 85%, from a reliable source. NIMAN RANCH is a great national brand if your local butcher doesn't have anything fresh.
i guess i'll let the experts weigh in on your question, because there is so much to know about beef!
edit: i see you are UK. though i doubt you could get niman ranch in a store, no doubt the world is at your doorstep regarding online beef purchases!