Just having a look now and it was the VRS-L batch that were disposed off very prematurely in 1979 at 6 years old.
Some of the early Leyland Nationals had relatively short shelf-lives. London Country - a significant user of the type who took over 500 of them onto their fleet disposed of the first ones in under 10 years. Yet the later ones (from about 1980) lasted much longer - well into the late 90s.
The very early 11.3m "Mk 1"s had a lot of foibles mentioned around here - the siting of the batteries being one issue, not the most robust of build - from about 1975 on Leyland had made changes to them and they were proving surprisingly robust. The last of them - the 'B' series Leyland National just seemed to go on and on, disappearing after Leyland National 2s had.