metres - small greek m
cm - greek epsilon E and small kappa K
mm - greek hee X and yiota i and lambda greek L

Thanks, gentlemen. I know that centimetre is εκατοστόμετρο but I thought they might abbreviate it to εμ. I'm glad I asked and didn't simply guess. I might have got a wardrobe larger than the house! 

Using Different Languages in your computer keyboard settings

For MAC users, the keyboard settings are part of your computer system preferences and the icon appears on the bar at the top of your computer screen usually showing the flag of the country whose language is currently set. Of course, mine is the Union Jack! I already selected Greek as an alternative language. Ι can see the keyboard layout in Greek by choosing "character layout" which pops up in the same bar.

For PC users using various versions of Windows, if you want to also use Greek - or any other language, below is a great website with instructions (free/gratis):
http://www.conversationexchange.com/resources/keyboard-language.php or make an internet search for any other website that gives free instructions.

Hope that is helpful.