From 1 March London Oxford Airport will have a listening out (frequency monitoring) squawk. The code 4517 to be used when monitoring 127.750 MHz (Oxford Approach/Oxford Radar). Under the system pilots add the code to their transponder and then listen on the frequency. Controllers can then contact any aircraft that they want to check or pass information to.
More information at http://publicapps.caa.co.uk/docs/33/InformationNotice2016017.pdf