(04 Apr 2016, 8:19 am)S813 FVK wrote Trains involved are units 150219 + 150249 and powercar 43160 - the vehicle on the other end is unknown.

It is believed that the pair of 150s were operating 2E68 (1339 Penzance - Exeter St David's) with the 43 believing to have arrived on 5A91 (1500 Laira Depot to Plymouth) but ran earlier so actually got to Plymouth at 1500 rather than departing the depot at that time. If this is correct, it means that the 43 set would have been in the platform 34 minutes before 2E68 arrive at Plymouth which it did at 1534.

http://www.bbc.co.uk/news/uk-england-devon-35954361

Even if the HST did leave the depot early, it was due to leave for Paddington a few minutes after the crash.