Dohh!!! wheres my reply gone to this thread then? i replyed to this last night and its vanished lol.
if the canvas is in good condition just use your normal wash shampoo, snow foam could stain the fabric if left to dwell for too long and as mentioned above you can strip the water repellency,
if its in bad condition and hasnt had any treatment for a while you can give it a quick soak in the snow foam aggitate,spray off well allow to dry and treat with this
http://www.i4detailing.co.uk/acatalog/Renovo_Ultra_Proofer_1.html, renovo do cleaners and dyes for soft tops also, highly recomended products.
once you've done that you find it much easyer to keep ontop of and it wont get as dirty either.