Dundee, Perth & London's BROUGHTY was renamed ABBAS in 1964 when sold to Stephenson Clarke and converted to an LPG tanker (where?) and sold in 1968 so that narrows it down to 4 years.
Even narrower, the Union-Castle at Smith's must be KENILWORTH CASTLE, last of the 3 Standard Fast type in the company to be in service, sold for breaking June 1967. The other two had been sold in 1959.