The FlightStats website can show you historic on-time performance data for a particular flight. You can get some of the information for free, but you need to register for free and login to get to the rest.

Might give some idea on whether the initial flight from Glasgow is likely to be on time. Regardless it sounds a bit tight for my liking.

For what it's worth, here's a link to the Caledonian Sleeper website (the service is recently under new management):
www.sleeper.scot