Any Mr. Percival headcanons?
Just one, and ironically, it's that there is no Mr Percival in my headcanon. Let me explain:
When the TV producers finally decided to introduce a Thin Controller in Series 9, they were faced with the dilemma of what his formal name should be. Series 9 as a whole was based on events which happened in 1977 - according to my own timeline headcanon - and during that time, the Skarloey Railway's general manager was Mr Peter Sam. Obviously that name couldn't be used - the potential for confusion with Peter Sam the engine was too great.
So after a bit of brainstorming, he became Mr Peregrine Percival. Over time, the character evolved into an amalgamation of all the real-life Thin Controllers - in the same way that the TV producers had previously rolled up all the Fat Controllers into one.
In the 'reality' of my headcanon, of course, this change never happened, so the SR remains under the control of Mr Peter Sam from S9-12 (1977-80), and Mr Roger Sam from S12-onwards (1980-onwards).