For a quick fix use Ajax professional for bathrooms or mould remover it may be called it is great. I sprayed it in our shower the other day and left it for a few hours and rinsed it off and it took most of the mould with it. If I had scrubbed it would have removed it all.

Unfotunantly its just a bleach type product so it will come back again, apparently the only way to get rid of mould forever is oil of cloves which you get from the chemist I think. Shannon Lush the author of spotless says this in her book. Maybe google it and see if anything comes up.
The thing with mould is you need to kill the spores and this is what ther oil of cloves does.
Good Luck, I know how frustrating it can be, the plumbers that put our brand new shower together did a terrible job so we have had mould problems from the start, in behind the silicon. It looks so horrible even though you know you have cleaned the shower etc.