Ok by total assyness, I seem to have alleviated the problem, but found another!
I noticed that the sub total wasn't showing in checkout, only the shipping and the total, so found in admin>modules>order total that "ot_subtotal" was not installed, hence was not showing at checkout, hence the message about the mismatched order totals :)

Hope that helps!

Now I have "merchant country unsupported" when i use my own visa card...!!! grrr...