That photo appears in "Dorset Steam" and is dated 4th July 1967, the year the Bournemouth line was electrified, so perhaps it was diverted from its usual route. It's the only photo I have seen of china clay traffic on the Weymouth-Westbury line.
Supporting that thesis is that 73018 was a Guildford engine in 1967 and would not normally have been seen on that route.