Only a quick update for today. I had an hour this afternoon and managed to fit - and do the initial testing of - an ESU Loksound L V4 in my GP35. I need to build a speaker baffle to better fit the speaker into the fuel tank on the bottom of the loco, which will be a project for another day.
The decoder fitting was surprisingly simple. I do still need to wire in the headlights for the loco though, but again - project for another day!
I'm happy - it works.