Thanks for the replies.
stevelup - I've done a continuity test from the plug to the mic pins and everything look fine there. Another piece of info I should have added to my previous post was that the mic picks up very loud sounds, e.g. blowing into the mic, but not speech or even shouting.
Jim59 - I was considering that but seeing as the headset was pretty cheap at around £80, even if I need to spend another £35 on a new mic I still think I've done alright...£115 for a spotless headset.