Hey Gordz - I've done Optifast before which I assume is similar to Optislim and they advocate that in addition to your shakes you're meant to be eating a certain amount of vegies/fruit don't they? This would surely increase your calorie intake? With optifast, even on the most intensive phase (3 shakes per day + vegies) it's designed to be around 700-800 calories per day.
Also if you're weight training, you really need to make sure you're having enough protein (I'm not sure exactly how much you should be having) because otherwise your body starts to break down muscles when you're on a low-calorie diet because there is not enough protein in your body to rebuild the muscles you've just damaged during the exercise. This means you probably need to take your whey protein at least once a day, if not more than once a day. Check Calorie King cause it tells you how much protein you should be having per day.
"Exercise has a profound effect on muscle growth, which can occur only if muscle protein synthesis exceeds muscle protein breakdown; there must be a positive muscle protein balance. Resistance exercise improves muscle protein balance, but, in the absence of food intake, the balance remains negative (i.e., catabolic)." Taken from http://www.ncbi.nlm.nih.gov/pubmed/11255140
(oh if this link gets removed, then pm me and I can tell you where I got the article from).