1. A method of using authentication apparatus to authenticate an access operation by a user by means of a multi-factor authentication process, the user having an associated access device and an associated notification device, said method including the step of notifying to the user's notification device at least one factor for use in the multi-factor authentication process, the method comprising: transmission by the authentication apparatus of a first factor of the multi-factor authentication process, in a notification message addressed to the notification device, said transmission occurring prior to the user requesting initiation of the multi-factor authentication process; detection at the authentication apparatus of initiation of the multi-factor authentication process from the access device; maintaining at the authentication apparatus a store of two or more valid first factors transmitted to a specified notification device; maintaining a store of two or more valid first factors in relation to a single user at the same time; locking by the authentication apparatus each transmitted first factor after receipt at the authentication apparatus for use in an authentication process, against subsequent use; receipt at the authentication apparatus, from the access device, of the first factor and a second factor, whereby the authentication apparatus is provided with two factors for use in said multi-factor authentication process.