The song is an english speaking woman, and it sounds very much like Goldfrapp, but I just can't seem to find out for sure who it is. The original song is the one i've posted from youtube, �guas de Mar�o - Antonio Carlos Jobim. It is sung in portugese and has been sung many many times.
http://www.youtube.com/watch?vXFsQmJMWf5Y
�guas de Mar�o - Antonio Carlos Jobim. This is the original song in Portugese
The tune is the Waters of March, a bossa nova by Antonio Carlos Jobin. It has been recorded many, many times - mainly by jazz singers. I have a recording from 1978 by Mark Murphy on an album called "Stolen Moments".
The British Gas version sounds to me like someone has specially written and recorded some different words for the advertisement.
If you google Waters of March it will bring up various recordings and you may find a version you like.
When Jobin sold this song to Coca Cola to use in a campaign in 1985 he was very criticized. Waters of March is considered one of the best songs in the world and for Brazilians it could not be used to sell Coke.
Now it is past and at least it seems that British Gas is using the original lyrics that Jobin wrote in English.
UKBR2008 - your hearing must be better than mine if you can hear the lyrics clearly on the British Gas Advert. From what bit I can hear I don't believe it is the original lyrics, I think it has been re written for the advert.
Here is the ad http://www.visit4info.com/advert/British-Gas-A ll-Over-Britain-British-Gas/56632