It's many decades since London to Doncaster took that long. Certainly since the HSTs arrived 40 years ago. I'm sure some nerd will be able to find a Deltic era schedule. Must have been about 100 minutes then.
Looking at the trains running today and tomorrow between Doncaster and Kings Cross, the fastest I can see is 1hr 36mins, with most being between 1hr 40 and 2hrs