Dettox mould and mildew remover, it is a green coloured pump action spray and it works like magic, totaly removes all signs of mould and mildew, but make sure you do not get it on your clothes as it will mark them. Get it from most big stores or hardware shops. Great for shower curtains too.
Do as carlp says then wash it in the machine , quickwash cycle to remove the dettox. You can get a similar one in poundland called astonish mould & mildew remover .Its very strong bleach , so use in a well ventilated place, outside preferrrably.
Same thing happened with my Outwell and I used diluted Milton fluid (in the baby aisle at the supermarket) - I actually used Tesco's own brand equivalent - which removed it and got rid of the horrible mouldy smell. It had been packed away for several months before we realised, but it has completely cured it. This seems to be a common prob with Outwell bedroom pods. Not sure why as we always make sure the whole tent is dry before packing away.