Good God, it's complicated... To summarise, the answer is 50 years, as the work was created before 1st June 1957. I've italicised the sections for you..
Term of Protection
The length of copyright protection for works is a potential minefield in international law. Applying it to purely national law is also extremely difficult in some circumstances. Each of the types of works eligible for protection under the 1988 Act has a different term of protection, further complicated by the amending legislation to the Act. Copyright protection always expires at the end of the calendar year concerned.
Crown Copyright, Parliamentary Copyright and Copyright of International Organisations
Within the United Kingdom, the term of protection offered by Crown copyright, Parliamentary copyright, copyright of Acts and Measures and that of international organisations is separate from that of ordinary copyright works. Copyright in a bill in Parliament, the
Scottish Parliament or the
Northern Ireland Assembly subsists from the moment a bill is introduced into the legislature to the moment it either fails to pass or receives
Royal Assent. The copyright of Acts and Measures subsists from Royal Assent until 50 years later. Parliamentary copyright of a literary, dramatic, musical or dramatic work subsists until 50 years after the making of the work. Crown copyright of published literary, dramatic or musical works expires 50 years after publication. Crown copyright of unpublished works expires the later of 125 years from creation or
31 December 2039. The latter provision is a transitional measure from the 1988 Act because that Act abolished perpetual copyright protection for unpublished materials. It is 50 years after the commencement of the 1988 Act plus the usual for expiration extension to the end of the year.
Crown copyright for artistic works is more complicated and is entangled in special treatment given to engravings and photographs. For artistic works made after the commencement of the 1988 Act, the rule is the same as for other works, 50 years after publication or 125 years after creation. An engraving created before commencement and published after commencement is in copyright for 50 years after publication. Copyright of an engraving created before commencement and unpublished expires at the end of 2039. Photographs taken between
1 June 1957 and commencement and published expire 50 years after publication. Photographs taken between
1 June 1957 and commencement and unpublished expire at the end of 2039.
Photographs taken before 1 June 1957 expire 50 years after creation. Copyright of international organisations subsists for 50 years from creation.