Manga High was set up a while back - Marcus du Sautoy is involved.
But it's been available just through a school registration.
Now you can register children individually from home - click on the JUKU tab in the top left hand corner.
It says it's "a BETA trial for 12 weeks and there is no cost involved to the early adopters". (No idea what that means, but it's free!)
My Maths-mad 12 yr-old is loving it.