If you live in Leeds, you're bound to know of a few Irish pubs. Get on the phone and ask about sessions that already exist. Force yourself to go, buy a pint, sweat a bit, and try to pluck up the courage to sing a song. If that doesn't work, get into a conversation with some of the musicians, tell them that you sing and they'll coerce you into singing. Enjoy yourself. Kick all the shyness and depression in your head into the middle of next week. (hopefully there will be another session going on so you won't have time to get depressed until the following week etc.)

If it's a "clicky" session and they have a problem with newcomers, f**k 'em. Find another one. If you're having fun i.e. feeling relaxed with people whose company you like (I hope you're not too polite to walk away from the ones you don't) You'll sing like a nightingale ... well O.K. I wouldn't want to sing like a nightingale either, (Something dangerous would have to be done to my genitals) but I think you get the Idea.

Don't take it too seriously, Music wasn't invented by psychologists, it's there to be enjoyed. Enjoy it.