I think its a lot longer than the Scotrail days since there's been significant local use of the West Highland sleeper. No where on the West Highland is actually that far from an airport now, even Fort William is only just over 2hrs drive from Glasgow Airport via the Erskine bridge, the Citylink Bus is scheduled for 3 hours but often arrives early. Another factor is since the West Coast Mainline Modernisation the lunchtime day train from the West Highland has a connection that gets in to London at a reasonable time with advance tickets starting at £50ish. Don't forget that the original Scotrail franchise specification didn't include the Fort William service in 1995 and it was only included following a legal challenge and strong political pressure plus an additional £700,000 subsidy. It being a largely tourist railway predates Serco management by some years.
. That's the difference between having the afternoon in the Highland capital or just the evening.