Have you tried "Dettox Anti-Bacterial Mould & Mildew Remover"? I have
found this product works exceptionally well. Just spray it on, and
within a few seconds you'll see it starting to work. Leave it overnight
if grout badly stained, there's no need to scrub normally.
Be careful not to get any on your clothes, towels etc, because it'll
bleach them. The 'trigger spray' often leaks mind, but I understand
they have redesigned it now (?)