Cleaning and caring for a vinyl roof

I have a white vinyl roof and wanted to know what's to best way to care for it?
I was told not to use anything on it by a car valeter but not sure that was right as its starting to crack and I can see hair line cracks on the middle of the roof, I clean it with just water and every month or so get a soft brush, bit like a big nail brush to give it a good scrub while hosing it down.
The roof is now 4 years old, I brought it from BAS international so its not a poor quality roof and I thought I would get a few more years out of it but its not looking good, I'll post up some pictures of the cracks when it stops raining.
The car is my daily drive and lives outside all year round come snow, frost, rain and sunshine!
I did not know if these vinyl sprays would mark/stain a white roof?
Any help would be great.

I use aerospace 303 for uv protection

http://bit.ly/1kNIto6

Also clean with Autoglym vinyl and rubber care

http://bit.ly/1jcJY15

2nd the 303.  I've used it for 10+ years on boat vinyls and it's second to none on plastics to prevent UV degradation.