Thank you for the reply, Andras.
Deactivating the plugins and switching to the default theme would not be feasible with this live site and the only way, that I can think of, to test if this is a theme / plugin conflict would be to allow several live transactions to occur while the main theme and plugins are deactivated; which would also not be feasible.
If we sent over a list of plugins that we are using would you be able to point out any that would be potential culprits?