After adding VAT, checkout consistently either £1.20 or £1.65 less than invoice total

Hi guys, I am new to this so bear with me. I have 2 websites, built by a freind of mine in India, both zen-cart, both e.commerce. I am not technically minded but my mate in India is pulling his hair out trying to solve a problem we have.

One is retail site and works fine, no worries. The other is wholesale and has VAT added at checkout. Although the calculation made by the site is correct on the zencart invoice, when the payment amount comes through to the payment gateway it is consistently either £1.20 or £1.65 less than the invoice total. It makes no difference whether the sale is for £10 or £1000 the discrepency is the same amount. The site worked fine before VAT was added as seperate component. There are two payment gateways on the site, one is paypal and one sagepay and the fault shows up the same on both.

Any ideas gratefully received. Many thanks, Chris Adby