Fiercely proud of his Celtic origins, actor David Tennant's research into his maternal grandfather's family uncovers the reason that his great-great-grandfather was forced to leave the Isle of Mull for industrial Glasgow. Travelling to Londonderry, David embarks on a difficult journey into the sectarian side to his family roots. David Dickinson traces his family tree tomorrow at 9pm on BBC1.

Shown last Wednesday on BBC1