(DA 2012/05/18)

There should be 17 of these according to the rubric. Please feel free to add the 17th in comments.

UPDATE: 5D spotted by DATrippers.com commenter JJ.

UPDATE TO THE UPDATE: 5D was spotted by Tim Aubrey first.

Across
12 Mad as a cut snake
15 High as a kite
16 Slippery as an eel
17 Red as a beet
18 Pissed as a newt
24 Calm as a millpond
27 Lean as a bean (this is the one that is reversed)

Down
1 Slow as a wet week
2 Dry as a Pommy’s towel [Grr!]
4 Blind as a bat
5 Old as Methuselah
7 Cold as ice
8 Quiet as the grave
13 Black as the Ace of Spades
14 Pure as the driven snow
22 Neat as a pin
28/23 Snug as a bug in a rug