From the Scottish Region in the 1970s :
The ScR had just five Sleeper Composites allocated long term, all the Anglo-Scottish ones (like the day coaches) were allocated to the relevant English region. There was a requirement for four per night, with one spare at Inverness, which covered just one car from each of Glasgow and Edinburgh to Inverness, passing en route. The stewards were based at both ends and did, like their cars, three return trips per week. The Inverness men (they all were) mostly were former Royal Navy stewards from Invergordon Navy base, who had retired from the Navy. The principal users of the Edinburgh car were Scottish Office civil servants, going to all day meetings in the North.
Lady who was a former colleague and then got a job at the Scottish Office described the procedure. In typical civil service fashion you had to attain a certain grade to get a first class single berth in the composite car, compared to the two-berth compartments in second class. This was the procedure to the extent that if the second class was full but there were first class berths available, you didn't go. However, given she was junior, she was booked second class, but the chance of there being another single woman to share the berth was pretty well unknown, so she always had a compartment to herself.
Meanwhile with the rolling stock, in best BR fashion the ScR ran the services fully with their own resources, both stock and stewards, and if there was a sudden vehicle failure at Edinburgh Craigentinny yard in the day, or steward illness, the Inverness spare would be positioned down in the afternoon, or a steward sent down by day train, rather than the embarrassment of having to wire York to borrow one of the several ER spare cars at Craigentinny. In fairness the ScR internal cars were fully traditional, steam heat, vacuum brake, and I think still on B1 bogies. The sleepers were only a small part of the consist, otherwise made up of Mk 1 compartment stock and several vans.