VAT tax calculation in shopping cart

We have a strange issue with our shopping carts.

We have the config set to do tax via R/3 but when the cart has items in it the VAT is calculated incorrectly, if we have 15% VAT on a net value SC of £100 it calculates it to £12 instead of £15 and in the case of 17.5% it calculates it to £14, this appears to be about 80% of the value in both cases.

We have checked R/3 (ECC6) and all tax codes are set correctly and indeed work correctly if PO is created in R/3 there are no discounts on the vendors we are using and it happens to catalogue and free text items.

Can anyone help

Tom