Thanks Paul,
I didn't really want to use discount codes either as it's a pretty clumsy solution but I've had a look at that option and unfortunately it's not going to work.
I can set up a discount code that applies to orders over a certain value but if I auto-fill it, it means it will attempt to apply it unsuccessfully to orders under that value.
Furthermore, the discount really needs to apply on the cart page and for some bizarre reason coupon codes are only applied on the checkout page.
Having spent some time defending Shopify as a flexible ecommerce solution, I am sadly being proven wrong and am now going to look for an alternative.
Thanks for your suggestions Paul.
Jamie