try fitting a cool tube or aircooled lamp to begin with, then ya wouldnt have this problem, HPS lamps do after all get white hot!,
dont ya even extract air from there or what, i find it hard to believe ya have heat problems in the UK, its 30 degrees centigrade here today! my room is 19 degrees plants 21.5 under lamp! 600 watts HPS, oh did i forget to menrtion i use cool tubes and aircon! ha ha
cheat aint I