Ok, I've just noticed that my admin tax has gone completely mad. And the gross value of products in my store are displayed incorrectly.

Here's what happens...

My tax rate is set to 17.5%. I create a new product and set the net price at £10. The Gross is calculate at £13.50! It should be £11.75.

I tried again, but entered the gross price first this time (£11.75). The net price was then calculated at £8.7037. It should be £10.

Also, the gross product price (catalog side) is completely wrong!

Both Display Prices with Tax and Display Prices with Tax in Admin are set to true. My default curreny has a rate of 1.00000000.

Any help would be great as I'm totally stuck with this one!