Can someone tell me how I would mend a top tier fondant cake after I have pushed the centre dowel in and ruined the middle? I understand that you can disguise it with flower etc.. but what if I want it bare?

1. Just use SPS and avoid dowels altogether.
2. If you must use a dowel for some reason (and I can't think of a single reason) then make a hole in the cardboard of the top tier and punch the hole (push a dowel) up thru about half the cake from the underside. Assemble yuor cake minus the top tier and drive in the center dowel, leaving just enough sticking up to be hidden inside the top tier that already has a hole in the cardboard and cake. If you've ever used the Wilton Tall Tier to assemble a cake, it's exactly the same process.
3. Seriously, just use SPS.