But when was that when did 150's and 153's first start running in the West Country?
O.K., I've got this from a book "The Exeter & Exmouth Railway" by Colin Maggs published by The Oakwood Press (ISBN 0 85361 430 X). Obviously given the title of the book, the comments relate specifically to services on the Exmouth branch, rather than Devon & Cornwall generally:
"By the 16th of May 1988 timetable, services were again worked by the older Heritage DMU's brought in to replace the Pacers which had been prematurely withdrawn the previous autumn."
"The Winter timetable for 1993-4, based on an increased allocation of the new Sprinter units, extended some workings to Milford Haven (!) and Gloucester for diagramming convenience."
But to muddy the water, there's also a picture of a 155 at Exeter Central working the 14.14 Barnstaple - Exmouth train on 6th April 1991, and two first generation DMU's crossing at the intermediate station of Topsham on 3rd April 1991. So it looks like the change over to Sprinter type sets, on this line at least, was a gradual process.
I hope this helps, up to a point.