Getting tax to display correctly on my invoices

[PerfectEye]

This is the bottom portion of an invoice generated from OSCommerce (currently testing some fake orders on the website i'm developing, just viewed this invoice):

Sub-Total: £99.00
Shipping: £0.00
VAT 17.5%: £14.74
Total: £99.00

The Sub-Total should just be the sum of the products ordered exclusive of any tax - but its actually currently the same amount as Total. The VAT (UK tax) is the right amount but the sub-total SHOULD be £84.26 - then it should get the £14.74 added to it, and then the Total should be £99.00.

Can anyone point out how i can get the sub-total to reflect the proper tax exclusive price? The prices are shown above in the main body of the invoice fine, so wondering why this is happening.

I can post the code for invoice.php if it helps?