MAYDAY RELAY is an acceptable term for use by sea craft, but has not found its way in to aviation RT phraseology.
It may not be 100% CAP413 as per PRadars post which I accept, but in my opinion it is better as it instanly alerts those listening that you yourself are not in peril, but someone else could be (satisfying the last sentence of 1.9). It is also internationally recognized and used.......in aviation and military.