Hi,

First just some clarification.

You say that on certain product combinations that a 6% rate is applicable. I assume that this is not a 'true' VAT rate, but some form of discount that is applicable only to your company ?

If that is true then you would need to actually apply the correct VAT rate for accounting purposes and to keep HM Customs happy, the difference (11.5%) would need to be applied as a discount. For this you would need to write an order total module (see something like the group discount module for inspiration).

For the other combinations which I am assuming are based on standard VAT regulations, this is still doable, although i'm not 100% sure its doable on an override basis.

I would start by looking at the cart class, there are some notifiers there that could possibly be used.

What you want to achieve (at least at the cart level) is change the tax class/rate associated with one or more products.

There are some examples in the wiki related to adjusting cart details based on the contents of the cart.