Quote the MAKE , MODEL and ALL the SERIAL NUMBERS found on the radio casing and possibly someone on here may be able to help.
However if you have already paid £20 for a code , why not go back and ask their advice. Perhaps you weren`t conned and the code WAS originally correct for the radio but since then it has been recoded to a different number.
Alternatively perhaps you aren`t entering the code correctly?
See info here
http://www.davidsonelectronics.co.uk/code_entry.htm
RegWorts, June 2008