Don't bother with a torch unless you really want one. The nights are getting lighter this time of year- I was on exercise at the weekend and I was part of a fighting patrol @ 11:30PM and we saw the enemy ridiculously clearly
Seconding Jcheese. Get one with a nice strobe function,really disorients people. I don't have one on my gun but I just use it to blind people If I'm detected and make a quick escape. You can get ASG flashlight mount which attaches to an M4 outer barrel IIRC. Or just tape it on.