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This work created by the United Kingdom Government is in the public domain.

This is because it is one of the following:

It is a photograph taken prior to 1 June 1957; or
It was published prior to 1974; or
It is an artistic work other than a photograph or engraving (e.g. a painting) which was created prior to 1974.

HMSO has declared that the expiry of Crown Copyrights applies worldwide (ref: HMSO Email Reply)
More information.

See also Copyright and Crown copyright artistic works.