Some outline timings on Steam Dreams website:
http://www.steamdreams.com/Executabl...en=0&Infants=0
It states that the train will be diesel or electric hauled between London & Crewe, with 'Flying Scotsman' taking the train from Crewe to Holyhead (& return to Crewe? - not clear)
I'm not sure where you intend to see the train, so the times below roughly cover the Welsh section:
Chester at 13.31, Bangor at 15.07 and arrival at Holyhead at 15.50
Then the return leg leaves Holyhead at 17.50, Bangor at 18.25 and Chester at 19.59.
Hope this helps,
Tony