This was a kids' drama shown on Granada in the 70s after lazy lunches on Sundays, if I remember rightly. It was about a mismatched pairing between a young, innocent, bespectacled schoolboy and an older streetwise boy, who both witness the murder of an invalid. They report it to the police but then the dead man shows up - but it's an imposter, of course! He appears to be involved in some Cold War espionage. Both the boys go on the run to avoid being caught by the bunch of Eastern European gangsters tat killed the man in the first place.