One thing nobody has mentioned is that in 1966, trains for London Kings Cross mostly used Leeds Central, not Leeds City.
I have a North Eastern Region timetable date 18 April 1966. From April until early June, the earliest train on Sundays was
10:28 Bradord Exchange / 10:52 Leeds Central, combining at Wakefield Westgate, and due Kings Cross at 15:06.
An earlier train commenced from Sunday 19 June, 07:12 Bradford Exchange / 07:36 Leeds Central, due Kings Cross 11:45.
To get into London earlier, you would need to travel overnight, in a non-sleeper service, 21:25 Bradford / 22:00 Leeds. due Kings Cross at 03:36. A later arrival into Kings Cross at 04:39 required changes at both Leeds Central & Doncaster.
In 1966, steam portions between Bradford and Leeds or Wakefield were still mostly steam-worked, usually by ex-LMSR 2-6-4 tanks or Black 5 4-6-0s.
Via the Midland route, there were sleeper services from Scotland, depart Leeds City at 02:28 and 03:10, arriving London St. Pancras at 07:30 or 08:30. From Bradford Forster Square, the only connection for either service departed at 01:27.
The first daytime service departed Bradford Forster Square at 06:38, and with a change at Derby, you could reach St. Pancras at 13:30. The next option was Bradford Forster Square 10:10 via Leeds, arrive St. Pancras 15:30.