Ahh, I never thought of it as a divider before. I've always thought of it as a limiting resistor and a pulldown resistor but by the looks of things it doesn't even matter which resistor has the biggest value as long as the voltage comes out right.

I'll have a play around with it so I can understand exactly how the maths is working and I'll test it out on my scope first to keep the PICs happy.

Ioannis: I can see how a coil would give a slight delay but I am only dimming lights with it and as long as I keep the timings right then everything works as intended. None of the coils I've used have been drastically out of phase. Even if they were then it's only a matter of adjusting the delay between the detected zero-crossing and resetting the outputs.