If you have a nearby Morrisons, they sell "Astonish" mould and mildew remover. It is only 99p and far better than the big name brands. I know, because my bathroom and layed up at winter Combo gets it terribly. Spray it on and don't wipe it off, just let it stay (smells a bit bleachy so open doors or lower windows, which also helps anyway). Maybe you will need another go, but mould spots just fade away after an hour or so. If the car's unused for long periods, leave the visors half-down as mould appears much quicker when they're tucked up.
Another suggestion, if you're mildly handy, is to consider an upgrade to the Vectra B's illuminated visors. Not difficult to wire up to the interior light and (worth fitting LEDs before installing) raising the mirror flap illuminates nicely. There are some threads here with pics. I'd do it on mine for Mrs Oz, but Vectra Bs fit fine on the Corsa C but not in my Combo Tour