Attribute price tax problem

Hi. I am setting up a UK store where all prices are subject to tax at 17.5%.

I have the options setup correctly and the store performs as it should. My products are set to display including tax.

THe problem is whenever I enter a price increase in an attribute. For example :

Lets suppose I sell a T-Shirt at £10 including tax in a medium size.

The T-Shirt product has a "Medium" attribute with zero price increase.

If the customer wants a large t-shirt however, they must pay an extra £1 (including tax).

So I set up another attribute ("Large") with a price increase of +1 to add a pound to the price.

However, in store, that attribute displays as £1.18 (including tax) rather than £1.00.

How can I alter the system please so that the attribute price increases I enter include tax?

Many thanks in advance.