Ford 5000 rds car radio/cassette security code?

Hi,

My mother-in-law has just had some work done on her car and the stereo is now asking for a code.

According to the Audio Manual, the serial number is M159780 and the code is 4885 but this code has been rejected.

She hasn't owned the car from new so perhaps the code was changed at some point.

Can anybody tell me what the code should be for this serial number?

Thanks in advance