I have checked the all line timetables for 1979 and 1982, and 1014 Perth to Motherwell does not appear in either.I believe it did run as a passenger train for part of the way; The Peter Watts publications 1H80 and 1H81 (detailing all loco-hauled passenger trains) show 1D50 1014 Perth-Motherwell, however in the 1982 edition it had disappeared, which corresponds to both the comments above. Why it ran at some times and not others, and when it did why only for that particular segment of the journey, is a mystery however !
In 1979 there was a 1210 Glasgow-Euston, which in 1980 and 1981 departed at 1205 to insert a pick up call at Motherwell and provide a very good connection from Perth. But in 1982 the Motherwell stop was removed and departure time from Glasgow reverted to 1210.
1014 Perth to Motherwell, together with the extra Motherwell call for the 1205/1210 Glasgow-Euston, look like an experiment that was regarded as a failure.