Are the RM not part of the RN?
In a word yes, and there was quite an interesting thread thoroughly amplifying this issue not long back.
Not too sure about MGD's simplistic view of the RM being the fourth emergency service, especially since the RM themselves would be the first to say that that honour belongs more properly to the RNLI, the Coast Guard, and the Mountain Rescue Service .... with the AA a long way behind, bless them!
Jack