Talking of fires, no one has mentioned that the introduction of the Mk 3 sleepers was delayed by the 1978 fire at Taunton in the Mk 1 stock. Design work on the Mk3s was held up until the report on the Taunton fire was available so that the Mk3s could include extra fire precautions. But I've no idea how much delay was caused......
I believe that in the 1970s BR looked at refurbishing some of the Mark 1 sleepers to life-extend them, and they could then have been used on the more marginal services such as Stranraer and Barrow which at that time were expected to last at least in the short term but which would not justify investing in new vehicles. In the event, I suspect that the Taunton fire, the asbestos insulation in the Mark 1s, and the difficulty of upgrading their interiors to anything like Mark 3 standards (even though this was successfully done on some other Mark 1-based vehicles such as the Southern Region 4-CEP EMUs) put paid to this idea, and also the withdrawal of some services such as King's Cross-Newcastle via Hartlepool meant that BR did have enough Mark 3s for the remaining "secondary" services (all of which have long since gone apart from the Fort William) after all.
That (along with the outcry from MPs who used it) might have been part of the reason why the Manchester and Liverpool sleeper survived a threat of withdrawal in the early 1980s (but finally succumbed a decade later, at the end of the 1991 summer timetable IIRC - see also the separate thread on the Manchester sleeper at
https://www.railforums.co.uk/threads/sleepers-at-manchester.169401/).
According to Wikipedia, BR also looked at retaining some Mark 1 sleepers to replace the existing purpose-built "Night Ferry" sleeping cars. That service could then potentially have lasted until the Channel Tunnel opened instead of being withdrawn in 1980, but that idea was also not pursued, probably for the same reasons. Also, the Mark 1s would presumably have needed considerable modifications to be able to run in France and Belgium (including fitting retractable steps for the much lower platforms than those at UK stations).