Hi Justin,
You would most likely need to contact the National Archives but the following may help.
http://www.veterans-uk.info/service_records/royal_navy.html (http://www.veterans-uk.info/service_records/royal_navy.html)
As for the number:
Officers Cooks and Stewards:
UPPER PAY SCALE PRE 25th October 1925
[Those already serving]
L......to L15101.
LOWER PAY SCALE POST 25th October 1925
[Those joining on or after this date]
LX commencing with 20001
Regards