I've been told by Z3I that applying Autoglym tyre dressing after a normal shampoo wash will do the trick and keep it looking black and keep it beading.
However, last night whilst looking through my cleaning products I found this in the wash-bag:
The instructions for the bumper and trim gel say it is excellent for restoring/protecting and giving a lustrous shine to black trim and vinyl parts.
Confused as to what to use, don't really want to damage the hardtop by using the wrong products!