Hi, my previous defender owner didn't have my radio code so a year after purchase when the battery was disconnected I too was left with a locked radio.
i discovered that after holding down the button 6 for a minute or two, the screen change back from locked to code.
I then entered the radio's 7 digit serial number starting with M into the website www.radio-code.co.uk under the Visteon page. After paying £4.99 with PayPal I got my radio code so all good now 😀