You're definitely right from a practical perspective,
No you are not!.
That transistor is all wrong. You should have the emitter going to ground and the collector / pull up resistor to the input pin.
If you enable the internal pull ups you can do without that 10K altogether.
One thing I'm really mystified about is about Resistor selection, what current should I be aiming for?
You are aiming for as much current as the pin can take, this speeds up the edges of the signal that can be slowed down by stray capacitance.
Sorry, I wasn't suggesting I was right, I was just agreeing with Riva's suggestion that I investigate the Sparkfun boards. Thanks for your help on this one, it's hugely appreciated.
Have gone and re-read some long forgotten things and have had another bash at the discrete circuit, does anything seem obviously wrong here?
Am I being daft, or would this invert the signal to the Arduino from the Pi by connecting the Arduino's Pin 2 to ground when the Pi's Pin 2 went high?
I understand that I can use the internal pull-up resistor, but at the moment, I'm just trying to grasp the electr(on)ics behind it all.