I do recall an announcement, maybe around 1980, of a 70 mph limit being introduced. As most night services then had at least one B1-bogied vehicle, always restricted to 90 mph, it made not a huge difference.
Bear in mind also that the overnight services were also fitted in with the large amounts of mail, parcels and overnight freight services, in a way difficult to appreciate nowadays. The northern end of the WCML was notably busier at night rather than during the day, and this was true elsewhere - in my own home town of Taunton there wasn't a lot of freight activity at all during the day, but at night services could be block-and-block apart, and the overnight passengers all had to fit in. In fact the fastest train of the night wasn't passenger at all, it was the 1 am from Paddington to Devon and Cornwall newspaper train, all vans but which ran at Cornish Riviera speed (in steam days it always had a 'King' in charge).