Discount percentage based on order value

Question

Please could someone share their wisdom on how to implement a custom discount percentage based on order total? I'm relatively new to Magento but have had a good look through the admin back-end of my site and these forums and can't find anything obvious.

Example:

• Order total £125.00 - Discount 0.0%
• Order total £200.00 - Discount 2.5%
• Order total £250.00 - Discount 5.0%
• Order total £300.00 - Discount 7.5%
• etc.