Does anyone know how to get Zen Cart to Pro-rata the VAT on delivery where you have made a mixed supply of standard and zero rated VAT items?

In the UK, you have to apportion the VAT on delivery charges based on the split of VATable items.

I've searched the forums but can't find the answer. I can't believe no-one has asked this before as it must be a fairly common requirement.

Both Roman Cart and Actinic do it as standard so I'm surprised I can't find a way to do it in Zen.

If it can't be done as standard (and no-one has made a mod for it), which module do I need to edit to modify the vat amount on shipping? Is it the Zone shipping module itself or somewhere else?

I'm using the latest version (1.3.9h) of Zen Cart and the Zones shipping module.

Thanks.
Bob.