Thanks for your reply, I don't relly understand how the Zen cart works and am just learning.
We use Realex for our payments and it is happening on all orders, they say it has nothing to do with them and we should contact our support people. Our customers are paying by credit card. As far as I am aware we havent had any recent changes to the site and the only add on we have is an "export email addresses" which has been there for over 12 months.
Any suggestions?