This is another previously unknown scene from a PFT that was on the BBC Store
Catherine Hall in a Play for Today from 1979 called
Even Solomon
http://www.imdb.com/name/nm0355396/reference
http://www.imdb.com/title/tt0771126/combined
The nudity is seen in a non-continuous jump-cut close-up, so potentially could be a body double. However her hair, hands and shirt all seem to match very well between shots (see row 2) so it is probably her.
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And as a bonus here is
Catherine Hall again, not-quite-topless this time, in
The Amy Johnson Story (BBC1/1984)
http://www.imdb.com/title/tt0133367/combined <-- (
listed there as just "Amy")
She only had a small part in it. Amy Johnson was played by Harriet Walter.