It would require creating two Shopping Cart Price Rules.
To have the discount levels start from specific subtotal we need to split the promotion into two separate promotions where second is disabling further rules proceeding OR both have constant start and end levels and are configured at the same priority (second approach is easier and I'll describe it but if you have more promotions that could be combined then the way with Stop Further Rules Processing could be the only one to make them work as expected).
Enough theory, let's create it.
First Rule Subtotal <= 1000 Pounds -10% discount
- Login to your admin panel.
- Go into Promotions > Shopping Cart Price Rules
- Create new rule: Subtotal <= 1000 Pounds then give 10% discounts
- Give the Rule some name and configure the generals.
Remember that LOWER Priority = Takes precedence.
Now select tab "Conditions" on the left side of your screen.
Leave the configuration as it is:
[If ALL of these conditions are TRUE :]
Subtotal equals or lower than 1000.0000
Now select "Actions" on left menu.
Apply: Percent of product price discount.
Discount Amount: 10
Second Rule
Subtotal > 1000 Pounds then give 5% discounts
Give the Rule some name and configure the generals.
Select tab "Conditions" on the left side of your screen.
Leave the basic configuration as it is:
Condition:
[If ALL of these conditions are TRUE :]
Subtotal greater than 1000.0000
Now select "Actions" on left menu.
Apply: Percent of product price discount.
Discount Amount: 5