My Freelander 2 has started to continually show a message: USB 1 Connect device. I have no idea why. There is the usual socket between the front seats, which I occasionally use to charge my mobile, but otherwise the socket is empty.

My guess is you have 'parked' your Audio on USB input. - it is looking for you to connect an audio device into that USB socket between your seats.

On the centre screen

Choose -
Audio
My Music
Source

I think that you will find that you have USB selected as your input.

Choose a different combination, --- mine is to my Smartphone via Bluetooth

or choose a another selection eg DAB, Radio. FL2 XS SD4 Auto 2010 2012-2017 (21k - 91k miles) (MY2011)
FL2 Metropolis SD4 Auto 2014 2017- (16k - 77k+ miles) (MY2015)
Metro in its 11th Year of (Extended) LR Warranty / Full LR Service History
(Expensive, but Trouble/Worry free - hopefully?)