I can say with absolute certaintly that the Varsity - and therefor most likely the Viking - had an SEP 1 Autopilot. In 1977 I removed a complete system one from a Varsity on the burning pan at Northolt for donation to Southall College of Technology.
Yes LFittNI, the Smiths MFS used on Vulcans and Victors was based around the military version of SEP 2 rather than the SEP 1. That Rate/Rate platform was certainly the way to go for tight attitude control. After leaving the mob and moving onto American built machinery, I was astonished to find the Pioneer-Bendix PB20 used in the B707 was a displacement autopilot with rate feedback. Whatever happened to British avionics? We were the cutting edge in the fifties yet lost the plot entirely in less than ten years.