I was on the 0940 from Penzance (changed plans and held my nose because I saw the London trains were messed up) and found out the reason for the disruption from this excellent announcement by the Guard after we left Camborne or Redruth (forget which):
This is a special announcement for the four young ladies and two young men who ran across the track in front of this train before boarding. We've already had one fatality on the railways in Cornwall this morning and the same could have happened to you....
Services were not normal by 0930 -- at Plymouth I saw that the 1044 to Paddington was expected at c.1204 and the 1200 around 20 minutes later.