The CAA have just published the standards for exactly that.
I gather there will be a number of HEMS going night ops. I believe East Anglia have announced similarly. All the ones I have heard of so far seem to be really operated by Bond.
I wonder to what extent they will operate in built-up Brum at night? I have not got around to reading the CAA bumph yet, so not aware what limits the CAA will impose in terms of congested areas.
http://www.caa.co.uk/docs/33/SafetyDirective2012001.pdf