I trust you have performed an intensive investigation with a team of expert engineers to determine that fact? The fact that you say most of it, means thet you think some of them are in a fit state to re use, would you be able to supply numbers?
There were apparently 120 SLEP and 88 SLE vehicles built by the production line at Derby, making a total of 208 vehicles.
53 of those vehicles remain in service with the Caledonian Sleeper pool.
10 of those vehicles remain in service on the Night Riviera.
That takes us to 63 vehicles, implying that there is something like 145 vehicles remaining
somewhere.
Large numbers of these vehicles have been dispersed to various preserved railways and could potentially be retrieved and returned to service if they could be repurchased.
However many of them do not appear to be in the
best condition.
I believe there were reports on this forum that the Mark 3 sleepers that are in store at various locations are not in much better nick.
Even if you could assemble enough sleeping vehicles to start this new service you will have to face the fact that you will need an enormous list of locomotives to be made available to run a service that runs from Plymouth/Bristol to Edinburgh/Glasgow. (With one split at Carstairs?).
Have you
seen the list of locomotives necessary to run the Lowland Sleeper, let alone the Highland?
For the special case of sleepers, it would seem best to use MU trailer cars, with a modified interior to include berths, be they conventional, or maybe japanese-style Pods. Since the MU trailer cars could be incorporated into a larger order of MU's the extra costs could be marginal compared to dedicated sleeper stock, and probably lower than ordering a Sleeper MU.
How on earth do you expect that the MU option will end up more expensive once you include the cost of the locomotives you need?
Even assuming that the traction package is the only thing missing and the carriages can thus be used to enable push pull operations you can't hope to manage that.
Say a sleeper service operating from Bristol to Edinburgh and Glasgow and the return journey.
You need 4 5-car multiple units, totalling 20 vehicles or roughly £30m.
If you want to run that service with loco haulage you would need two locomotives for the main line section from Bristol to carstairs and a third locomotive to handle the operations of the service on whichever leg doesn't get to keep the original locomotive.
That is £9m before you even start on the rolling stock.
So you have £21m and you have to buy 20 vehicles, that must include 8 driving vehicles, the only thing you save is the cost of the traction packages.
At best you save a pittance, and then you have to include the operational costs of operating this fleet of locomotives when you could quite easily use multiple units that will use a fraction of the energy and have so much lower maintenance and track access costs that its not even funny.