I'd start a business of running night services right across the network by re-instating all the previous withdrawn sleepers and taking over everything that operates long distances between 2201 and 0529. Stock would be refurbished MK1s and MK2s cars, 47 or 85/86/87 hauled. Livery would be a dark blue blending into dark orange with the crescent of the moon to represent night time.
I'm not sure that many of the remaining Mark 1 and Mark 2 coaches would be suitable for conversion to sleeping cars. There were no Mark 2 Sleepers, and there are only about half a dozen surviving Mark 1 Sleepers mostly on preserved railways. That is, unless you were to go for couchette-style accommodation (with four-berth compartments convertible to seats by day).
I would suggest an open access cross-country Sleeper service using the Mark 3 sleeping cars displaced by the new Caledonian Sleeper stock (plus possibly some ordinary Mark 2s and/or 3s, either as seated coaches or converted to side corridor compartment layout and used as couchettes as described above. In fact, on a route where proper sleeping cars would not be economically viable, there could still be a potential market for couchette-only trains).
One train could run from Edinburgh and/or Glasgow (dividing and combining at Carstairs, or starting from Glasgow and running via Edinburgh) to Plymouth (or Penzance) via the West Coast Main Line to Birmingham, then via Cheltenham and Bristol, and the other starting from Newcastle and running via the usual cross-country route via the East Coast Main Line, Sheffield and Birmingham.