Sorry for extreme delay in replying as my laptop is goosed. Hopefully new one in the new year. The simple & official answer is this: No space available & no reservation = no travel! There is no standing on the sleeper. Unofficially who knows?
Do you need a reservation to board the Caledonian Sleeper?
If 1st Class berths were sold out
Standard Class berths were all occupied
And seated Sleeper place all sold out
What would they do if you boarded with a valid ticket? Would you just have to stand in a vestibule all night?
I was thinking of boarding the southbound lowland sleeper from Edinburgh on Sunday night with a Super Off Peak and no reservation and seeing what happens... all the seated and standard berths have gone, so it's just first berths iirc; I've got a return un-used Super off Peak Edinburgh to London Terminals and I'd rather not pay anything 'extra'.