There is a statutory defence under paragraph four of Section 172 RTA 1988, and we suggest that you read these pages very carefully, as there is a similar case to yours at "providing a list of possible drivers".
This forum topic covers this topic in some detail.
This forum topic will take you to an example of how this defence may work in practice.
When preparing your defence, you may also find the case of Sheldrake -v- DPP in the forums useful.