Hello Antares, welcome to the best part of the forum.
In order to rectify something you have to understand what it is, thereby you can find its weakness in this case.
Blanket weed is also known as string algae, your biological filter and 13 watt UV can only remove what passes through them.
String algae does not go anywhere near a bio-filter or UV, that is why a bio-filter and UV is useless against string algae.
There is only one guaranteed way to remove it and that is with

but you only need one.
There are lots of "potions and gadgets" that claim they will rid your pond of string algae but what works for one pond can not be guaranteed to work for all ponds, but no one can find out why, so until they can, its back to the good old stick. Even then there are various forms of "stick"
oh, and be careful where you swim, or you could end up like
One more thing."Thanks" goes a long way.