Discount Module for OS Commerce Installation.

I would like a discount mod applied to an installation of OS Commerce. The module should do the following: All Transactions on Internet = 5% as a standard discount.. Then, based on a rolling 12 month spend: Over £200.00 but under £400.00 = 6% £400.00-£600.00 = 7% £600 to £800 = 8% £800 to £1000 = 9% Over £1000 = 10% These discounts must be based on total spend (bait value) on the 12 month rolling period and are not based on individual order totals. Therefore, if someone places 10 orders of £101.00 through the 12 month period they qualify for the 10% discount as their total spend for the period is £1010.00. The mod needs to automatically calculate the 12 month rolling spend to place customers into the standard 5% discount band or the 6, 7, 8, 9, or 10% discount band. If they drop out of their band they must be automatically moved down or if they spend more they must be automatically moved up to get the allocated discount.