Self-certification at work normally allows you to literally self-certificate yourself as having been ill for a certain period of time without a Doctor confirming it. After that you need a certification from your Doctor. If you went off ill on Friday last (i.e. were off ill at some point on Friday 27 June) and were still off with the same illness on Friday coming (i.e. 4th July) then they can ask for a doctor's note. If you are still ill, you certainly should not return to work. If there's a charge for a Doctor to give a hindsight note because you haven't been to see one while you were ill, investigate whether your employer will pay. Do this via your Trade Union if you are a member, which I hope you are :-)