Mildew comes simply because of moisture in the air, which always happens over the winter. Hence, two things to do.
One, instal a dehumidifier over the winter months to dry the air. Two, instal some 'greenhouse' style heaters to keep the temperature above 10c or so.
This will stop it from forming.
Getting rid of the existing mildew is simply a matter of wiping it off with a damp cloth. Whatever you do, do not use a cleaner of any sort whatsoever - they will almost certainly discolour and/or damage the surface material.
I have used the above method for years, and it always works.
Neat Milton on plastic material does the trick.It will kill the spores and lessen the occurance in future..beware the bleaching effect of Milton on fabric though. The cure at source is of course to dehumidify. David