A lot of people use the AG roof cleaner and sealant. You will need a soft brush to work it in and get all the crap out first. Best not to rub too hard, but just gently agitate all the crap out. Wash it down, find any more bits that need a bit more work and once its spot on, then consider sealing it. Its best to get absolutely everything off before sealing for obvious reasons.
e,g, http://www.amazon.co.uk/Autoglym-Cabriolet-Fabric-Hood-Cleaning/dp/B006R4HEBG