Found a mobile whilst on holiday in the Chilterns. We can only access a screen which states, "This phone has been lost. Please ring *****." Tried to do so, but being informed that the number is not recognised. Any ideas?

eBay.

Not sure if the phone will accept incoming calls, but if so, you could keep it charged up for a few days and see if anybody calls it.

use the sim card in another phone. See if there are phone numbers on it.