Hi MiniMoodle - and welcome to MINI2.com
The most recommended product on here is the AutoGlym fabric roof shampoo and protector. They come in a pack together priced between £15 & £20.
To use it, pick a nice warm sunny day!! Spray on the shampoo and work in using the foam pad supplied. Rinse off thoroughly with a hose, using your hand to brush the suds out - rather like bathing a dog!
Allow to dry - this can be speeded up by soaking up excess water with a Chamois!
Once dry, spray on the protector. Avoid contact with glass, trim and bodywork. Mask these areas off if possible. Allow to dry overnight
The water beading once you've done this is spectacular!!
Hope that helps!!
Jon