[edged]

Thanks for the welcome both! 

I've just been doing some bench testing with an old Arc Air powerbase and a multi-meter. I think my assumption that a +5v pulse to a certain pin on the sensor connector is the problem, it's not how the Scalextric system detects the guide blade; instead what seems to be the case is the following:

red = gnd
white = +2v with no car on lane 1
white = 0v with a car's guide blade blocking the IR sensor on lane 1

Back to the instructions for the track interface for 'negative going' - I think this is more likely the right setting to fool the Scalextric unit...