It was in done in 1989 for a financial institution known as the Abbey National building society. The little jingle, called "Abbey Endings," is about all the happy little things you do do with a loan from Abbey National. Bart is surrounded by little kids who sing along with him, the charm of which is what clearly caught on with the public. Especially since the words are a bit to make out, but the warmth comes through. The ad, which is fairly won a Silver award at the British Television Advertising Awards.
Indeed, it became so popular, that an extended version of the song was released as a single, with Bart and the kids, and became a hit on its own.
Here's the ad that started it all.