After a flight of some two hours, made for the purpose of testing radio navigation equipment, the aircraft returned to London-Gatwick Airport, U.K.. Before, and during, the flight, undercarriage operation was normal, as was the functioning of the undercarriage light and horn warning system. During the approach, the undercarriage extended normally, and the green lights indicated that it was locked down. Some 50 yds. after touch down, the aircraft began to sink on the starboard side and the captain noticed that the starboard green light was no longer showing. There were, however, no reds, and the horn was not sounding. The starboard main undercarriage leg eventually retracted and the aircraft swung off the runway to the right, on to the grass.