Have you tried mould spray! that stuff is amazing, don't know what they put in it but gets rid of all those nasty black or orange mould spots that never seem to shift. I discovered a nasty black mould patch on an external wall. I left neat bleach on it overnight and it didn't do anything. A spray of mould spray and it had all disappeared in a couple of hours.
This stuff
http://www.jtspas.co.uk/hg-mould-spray- ... oChe_w_wcB
think I got it in robert dyas, or maybe in sainsbury's...