Roy,
I have no personal knowledge of this place, so I can only go by the directory's. They put the Riviera at 197 Soho Road, next door to Broadmead's that stood at 199-201 Soho Rd on the corner of Nineveh Road. It opened c1962 and closed c1966 during those years it expanded into 195 as Riviera Catering the premises closed when the corner of Nineveh Road was demolished and replaced with new shops.
Anyway that puts the Riviera at No 197 right behind the traffic light next door to Broadmead's