I've always just scrubbed it with soapy water and given it a good rinse afterwards. My car is just over a year old and I don't feel the need to use expensive conditioners on it yet. My roof is green, so I suppose I get away with more than a light colour like yours.
Most of the guys over on XKEC use Autoglym roof restorer, Shampoo and Conditioner.
Jaguar sell thier own equivalent, which is probably Autoglym but under their own brand.
The JDHT were selling this at bargain prices recently. Their web site seems to be down this morning so I can't check.
http://www.jdht.com/index_intro.html