Advice on employment contract?

I am currently signed off work by my doctor under stress. The reason is that my employer has changed my job description and will not change it back to what I was doing, infact they have brought in someone else to do the role that I was doing. Anyway, I want to hand in my notice. Is it possible to give my notice to my employer without working it but supplying them with a doctors certificate for stress to cover the remaining weeks? For example, I have already been absent for 2 weeks and will get another doctors line for the last 2 weeks to cover my notice period (I will actually be backdating it from 28/09/06). Can anyone please advice? No smart a r s e answers please as this is very important to me!! My notice period is 4 weeks in total.