Quick update on this, many thanks to SuperDynamicCow and of course the OP for the controller zip, that has worked fine. Apologies for the delay in thanks, the Steam Deck was a Christmas present so it was only today that we were able to retry.
We do have a secondary problem in that Steam + X isn't bringing up the keyboard, but to save me making the same mistake I went through this thread with a fine toothcomb, and I'm 99% it's because we're coming from desktop mode. Fingers crossed a quick reboot straight into gaming mode will resolve that one.