The recent "smoke free" legislation doesn't apply to open station areas.
However ATOC (the Association of Train Operating Companies) and Network Rail are using a long standing railway bye-law which makes it an offence to smoke anywhere, on railway property, where a 'No Smoking' sign is displayed. (So, yes, it's most definitely enforceable).
That information is made clear in this Network Rail press release:
http://www.networkrailmediacentre.co.uk/Conten t/Detail.asp?ReleaseID=3125&NewsAreaID=2&***** ***********=2
If I was still running a busy railway station (as I used to do) I would be extremely happy not to have to get someone to sweep up all of the dog ends every half hour.
Chris