That I didn't know, being only an occasional user of VT (about once a year to each of Birmingham and Manchester). How much would it be to upgrade on a Saturday from Manchester to Euston? I've got a 20-odd quid ticket for a date in April. However if it's not too busy I probably won't bother as I'm likely to fall asleep anyway.
All Weekend First upgrades on Virgin Trains are £15 throughout the VT network.
This means that even if you wished to upgrade Glasgow-London, it would still only be £15, with Manchester to London being the same price.
I think it's very good value, especially when you can benefit from the weekend at-seat services on Pendolino's.- I think it's offered on most day time Pendolino services apart from those north of Preston, although i'm not 100% sure on that.
So why don't they mention this on their website?
My source-
http://www.crosscountrytrains.co.uk/Ticket_options/Weekend_upgrade.aspx
"Also note it is not possible to purchase an upgrade in conjunction with all types of ticket, customers holding Standard Advance tickets cannot purchase the First Class upgrade. We would suggest that you purchase an Advance First ticket for your journey if you are looking to travel in First Class"
The only time I've personally double checked this with a XC TM was Christmas 2008, so things may have possibly changed, but I doubt XC would revert back to the original rule it changed itself.
I'm very confused as to why there appear to be 2 different versions of the weekend upgrade offer on the website though..
