Fader Wing with Cobalt 20 - setting Master page?

Hi all,
How do I set which master page is on the external fader wing (2 x10 fader) when it's attached to the Cobalt 20?
It's defaulting to faders 41-50, 51-60, (i.e the 3rd group of 20) but I can't find anywhere to change which page is set to that bank.
Any suggestions are welcome!

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