Do I need to lock chickens up at night?

Hi,
My chickens live exclusively in a secure run.  It is a wooden cage covered on the sides and roof in weldmesh and placed on brick foundations.  The coop is a small 4-hen one, similar to the one attached.  Currently I lock the chickens in the coop at night.  I do not do this from a safety point of view, more from a health point of view.  I feel that if a fox could get into the run they would have no problem getting into the coop. So the main reason for locking them up is to keep the draft out.  However, it would be nice to go away for a night now and again.  So I was wondering what people's thoughts were on whether I need to lock them up at night or not.  Finding someone to lock them up last thing and let them out first thing is not all that easy!
Many thanks for your thoughts and comments.
Chris

   Personally i think your putting your girls at risk leaving their door open, you could also get one of those automatic door openers that are on timers, might be safer.   They are your hens and it is at the end of the day up to you.  nicky

Thanks for the comments, really appreciated.