I've popped my knee a couple of times in the past myself, so I really need to get around to strengthening the area.
Can you recommend any good exercises to help?
Currently I run, do leg-extensions and the odd leg-press.
Not sure how good lunges might be for your knee but are a good exercise for balance and a bit of core work.
If I were you I'd be doing leg presses (ideally squats) and then the odd bit of extensions. Not the way round you're currently doing them as I've heard extensions aren't brilliant for knee health. Target your legs as a whole, not just the knees