I've just put a post on the Britax page - we have the maxi-riders for both the girls and both are above 18kg - I just assumed while they were within the height of the 5 point harness that that would be the safest option... Oops.
I think we might be swapping them to the lap sash - I wont be buying the H Harness as I had also heard (and thanks for the reminder) that it was safer to just use the lap sash only with these boosters.