strange fly on my chillis!

I have a few strange flies in my mini greenhouse - they have been there a while and so far I can't see any harm done but they seem to sit on my chillis so I thought I'd just check as one has bites out of it's leaves and has fallen over (all the rest are in perfect health) they are about the size of a normal house fly though with slightly longer bodies and they are yellow. Am I okay just to leave them alone? I was hoping they were eating aphids!