I also seem to recall that a few of the long-gone secondary Sleeper services, such as Paddington to Milford Haven, were mainly newspaper and/or parcels trains and were thus mainly formed of NPCCS but with just one or two sleeping cars in the formation.
Sectorisation did for most of those, as any train carrying passengers was deemed to be "prime user" for the relevant passenger sector, even if its
raison d'etre was the parcels sector. This meant most of the costs were accounted for in the passenger sector's budget, with the parcels sector only paying the marginal costs of the extra parcel vans/ mail coaches etc.
So to improve the Inter City (or NSE or Provincial) bottom line, the trains were made non-passenger. The trains still ran, often still with a BCK or similar in the formation as there were not enough BGs around, but now the parcels sector footed the whole bill.
And cheap overnight travel became a thing of the past
(BCK - brake corridor composite, BG - full brake (gangwayed))
(second class compartments were the favourites as on a quiet train you could lie down across the seats like a couchette - not possible in (declassified) First as they had armrests)
