Thanks Guys
Just been doing some net surfing and looking what boat owners use, many say not to use T-cut or abrasive cleaners as its too harsh but i reckon thats just due to the high gloss finish of boat fibreglass, they do say to wax it after cleaning though, the caravan boys say to use caravan cleaner such as fenwicks from Halfords to shift the grey streaks etc, also read that someone had good results using MER and i have some of that in my garage already, so i reckon i will spot test a few items and see which gives me the best result and use that.
Also i used muc-off on my T25 roof as that was very grotty when i got it and that cleaned it up a treat and i have gallons of that stuff in the garage but i don't want to use anything too harsh that might bugger the roof up :|