There is on just after security to the right in the north terminal and it's been there for at least 5 years.

Also... "Responding to Prof Colbourne's comments, a spokesman for airport operator BAA said drinking fountains offering free water were already installed at Heathrow, Gatwick, Stansted and Southampton airports."

This is from 2002... http://www.dailymail.co.uk/health/article-355500/Free-water-...