The Meritorious Service Medal is awarded to individuals of the rank of substantive sergeant and above who hold the Long service and Good conduct Medal and have completed 20 years qualifying service. It is awarded to those who have shown good, faithful, valuable and meritorious service with irreproachable character and conduct.
On a personal note, I was honoured to be awarded he RAF MSM in the 2013 New Year's Honours List and collected the award last week (14 Mar 13)