It's a long-shot, but these people sell old GWR timetables, you might just hit lucky and get the one you want:
Nevis Railway Bookshops, 01635 200507
Edit: I think the one you need is available on here:
http://britishrailwaybooks.co.uk/wtt...er/western.php, though it will set you back £18
His contact page includes an evening telephone number:
http://britishrailwaybooks.co.uk/main/contact.php
Good luck.