...each to their own and theres no right or wrong, but i would strongly advise quitting your day job right now.
If its making you that unhappy then theres processes any decent company should have in place, employee services, a little chat with your line manager, some time off....
Considor retracting your notice James...
Thanks a lot for the advice, but - Nope... sorry - I am off 
Fair Play and Good Luck mate, by the way what sort of reaction did you get to your notice letter and what reasons did you cite if you dont mind me asking?
My boss is not in at the moment.
I told my boss last week I was hanging on by a thread, and was really unhappy.
She told me she feels the same because of senior management, and her hands are also as tied as mine..
She asked me not to go, told me I am very valued and she will be very upset to lose me...
She know's it was on the cards, because I did tell her I was considering leaving and I told her truthful reasons.
Its going to upset her because we have 6 or 7 years of working history and usually work well together, but I have no choice.
I'm not looking forwards to when she opens her inbox tomorrow...