I would place this photograph in the period 1925-35. It is difficult to be more precise. I have chècked pictures of Okehampton throughout the twentieth century and the only indicators we have here are
1. Fashions -very much inter-war and 1920s rather than 1930s, I'd say.
2. The old victorian gas lamps are in course of being replaced by standard SR concrete ones. This happened all over the Southern.
3. The coaching stock visible was standard on all the Okehampton routes between the wars -3 and 4- coach sets.
All the best, Roy