Interesting - I've been looking through Tom Baker's stories (http://en.wikipedia.org/wiki/List_of_Doctor_Who_serials#Fourth_Doctor) to try to guess which one this might be, but very few of his were set on Earth around that time. The only one which even nearly fits the bill would be City of Death (http://en.wikipedia.org/wiki/City_of_Death) (autumn 1979), which is actually set in Paris, but does involve a lot of running-around scenes which are clearly designed to show off the setting. At three years old, we might well have got this confused with London - do you think that is what you're remembering?