Took a look at the drain screw Rodge and your right, it is made of soft metal and refused to budge. Not wanting to strip it I was about to set about removing the carb to investigate what was more than likely a stuck float, maybe a bit of grit, when reckoned if i turned the fuel off and started the bike this would empty the float chamber of fuel and hence move the float. It did , bingo ! It's now not leaking but i'm under no illusion the problem could return, but it's better than having to strip it all down!
Thanks for the input.