Pilot Pete, I agree. "Declaring an emergency" is for Hollywood stars in action movies. It means nothing.
If there is a problem don't flute around. Mayday gets peoples attention sharpish and gets you what you need immediately. When your back on the ground afterwards you can discuss the rights and the wrongs.
I spend most of my working days in Italian airspace, communication is a major problem and I have experienced that any questions that deviates from standard RT usually has the controller stumped. Our company demands a PAN if EFATO, and I play that game in the sim, but if it happens for real I will use the word MAYDAY!