I am hoping someone can give me a magic method for the removal of mildew and the marks from fabric without removing the fabric from the boat?
While there are many methods of killing moulds and mildews in situ they won't remove the stains. I think you knew the answer when you asked the question, unfortunately the only way to remove the stains is with laundering. once the fabric has been removed that will give you a good opportunity to spray or wash the inside and kill any other remaining spores.
On the downside whatever you do will be no guarantee that the problem won't return. Fungi spores love warm moist places, lots of ventilation is the answer to controlling them.
As LooseChange suggests, remove the conditions that encourage mould to grow. But once it's there oil of cloves is your best weapon. Mix a 1/4 teaspoon of oil of cloves per litre of water, and mist on with a spray bottle. Leave for 20 minutes and wipe off. Spray again and leave. It will take 24-48 hours for the mould spores to dry and drop off.
