Problem adding two materials to an object

I thought it was possible to assign a material to a face in edit mode, different to the material assigned to the rest of the object. I can’t seem to do it though. Whenever I select a material it is assigned to the whole object in edit mode, even though I have only one face selected. Must be doing something wrong but I can’t work out what. I did solve the problem by separating the face as a separate object, assigning the second material, and then joining the objects back together again. Seems a bit of unnecessary work though.

what you are doing, is changing the material on the object, ur not adding new materials.

click the pluss button in the image, this will assign a new material TO the object.

OK, thanks. I knew it must have been something simple.