Thanks for the tip, I didn't know about setting it up as a Mackie. Having done that, the faders do control the Cubase faders they're meant to control, but they still also output a high G# note and a pitchbend signal as well as controlling the faders.
I have since heard back from the shop I bought the keyboard from, and they think it's a fault, and have offered to swap it with another unit once they have one in stock.