Black mould spots won't shift!

I have a Freedom Microlite and it has been under a farmyard shed (open one side) for the winter.  My mistake was putting it there without cleaning it.  There were already some mould marks on it from the Autumn rain and some leafs got on it - yes mistake No. 1.  Wasn't particularly worried till yesterday as was reassured Fenwicks cleaner would remove all marks.  It hasn't!!  I used the Motorhome cleaner (local supplier was out of the Caravan cleaner but told me it's just the same) and rinsed with Fenwicks Bobby Dazzler as per instructions.  It took a lot of the superfical grime and mould off but still left with black spots which appera to be embedded in the surface.  Tried putting the cleaner on almost neat.  Don't want to scrub with a hard brush as will scratch the surface.  Any ideas or suggestions welcome.