I am pleased to confirm we have now launched "Quota Bonus"
The concept is simple, and applies to Home::1 and SoHo::1 on all levels including terabyte.
You start your billing month with your quota as normal, but get an extra bonus that is half of the unused quota, if any, from the previous month.
This allows people to build up a reserve and allow for occasional higher months without needing top-up.
Thanks to all of the customers for the feedback on my blog posts on this. --Adrian.
P.S. yes, it is sort of cumulative, see examples on http://aa.net.uk/broadband-quota.html