I tried the Fenwicks caravan cleaner (available in Halfords) a few years ago but it seemed to take the shine off of the upvc which I didn't like.
I mainly just use pure water, did a minger yesterday afternoon that came up well.
If I use a detergent I usually just use Flash all purpose cleaner and might mix it with Ecover washing up liquid to thicken the mixture up a bit if squirting it on the brush.