Incorrect UK tax calculations on admin invoice. What's going wrong?

[Dunk]

We're based in the UK so charge 17.5% VAT on everything we sell. All items and shipping are therefore taxable goods.

Calculations work fine on the online store where all displayed prices include VAT, however, the print invoice doesn't seem to be calculating VAT properly, see the attached image.

Example, we sell a steel clip for £0.51 (ex VAT). The invoice gets the single unit ex VAT price correct, but then lists the inc VAT price as £0.51 too!
Similarly, it gets the total ex VAT price correct (in this case 10 x 0.51 = 5.10) but lists the inc VAT price as £5.10 as well!

The sub-total at the bottom of the invoice IS correct though, as is the final VAT figure, and the overall invoice total, it's just the calculations on each row of the invoice.

Any ideas? Have I forgotten to enable something?