In April 2010, the sickness certificates, Med3, Med5, Med4 were replaced with "Fitness for work" notes, still known as Med3 (04/10).
These notes give more flexibility, both in terms of the Dr completing them (eg can now be over the 'phone were appropriate) and stating what you may be able to do. There are 2 categories, "you are not fit for work" and "you may be fit for work taking account of the following advice".
For example, if your job involves stacking shelves and you have a shoulder injury, the GP can state that you "may be fit for work taking account of the following advice", eg should avoid heavy lifting and working above shoulder height, for say, 6 weeks. If your employer is able to find you suitable work, you can remain in work. If they can't then the note will be used as if it had been completed that you were "not fit for work"
Please note that you do NOT need to be signed back for work, nor do you need a Drs note for less than 7 days absence through illness. For the 1st 7 days, you need to complete a SC2 form, which is available from your employer or can be dowloaded from: