I seem to recall seeing the adders once - but it could well be the Armenian Viper I am recalling (another species which has gone in the past year, incidentally) as the two species are akin in appearance.
Thanks to the links posted by tennisfan, this is the full list of snake species which London has gone out of in the past year, along with how many individuals were held at the start of 2013. No snake species appear to have been acquired
Broghammerus reticulatus - 1,0,0
Morelia spilota spilota - 1,1,0
Morelia viridis - 0,0,2
Thamnophis sirtalis tetrataenia - 0,0,7
Lampropeltis getula californiae - 0,0,1
Bothriechis aurifer - 0,1,0
Bothriechis lateralis - 2,2,0
Bothriechis nigroviridis - 1,1,0
Bothriechis schlegelii - 3,8,0
Bothriechis supraciliaris - 0,0,4
Parias flavomaculatus mcgregori - 1,3,0
Trimeresurus trigonocephalus - 1,2,2
Vipera berus - 0,0,2
Vipera raddei - 1,0,0
So that is a total of 14 snake species lost in a year - in terms of individuals across the whole reptile collection, they went from 296 animals in 2013 to 209 in 2014.