HELP ME :( VAT calclation error

Hello, hope someone can help us out here! We have come across an order which has calculated the VAT incorrectly. We cannot for the life of us figure out why. We have successfully processed 843 orders so far without this problem. It is a Zen Cart 1.5.1 website. To illustrate I have uploaded a screenshot of what is seen on both the shop side and the admin side, and how I have worked around the issue for the customers benefit. No changes to the site's files have been made for months. :ohmy:

Oh my, I feel I should at this point add that I know that the system has added the values correctly. I have reproduced this in a spreadsheet and on a calculator and the values match what Zen Cart outputted. But our Sage Accounts system outputs £0.01 less and our accountants say we have to go with what Sage outputs: #groan

Feel slightly silly and at a loss on what to do now.