Pure clean water, and lots of it, thats all you need to use.
For any stubborn marks or stains use a product especially designed for the job, I find 303 Aerospace Convertible cleaner works well.
Clean thoroughly and proof x 2 yearly.
Try to use a PH neutral shampoo, and avoid products that say " contains wax", getting on the fabric.
Remember to include vacuuming as part of tour maintanence program, especially where the hood folds.
I was recently stopped the other day and asked to clean a mini with a fabric roof, "I dont want anything fancy she said, I only go down the road and pay £10".
The fabric hood had started to rot through in numerous places!!!!