... but it may be of use to others too.
Virgin trains are actually pretty reliable at giving you your money back if they run late*. You need to be 45 minutes late or worse, be able to remember enough about your journey to show that you were 45 minutes late or worse, save your tickets, and fill in a form.
The form is a little tricksy to hunt down. So having gone to all the effort of finding it about 6 times now, here it is.
* And as far as I can tell it generally _isn't_ their fault when they run late. The main things that give them difficulties, based on a year of commuting, are fire, flood and network rail, which are formidable adversories at the best of times.