JMA,

You cited:
Op Manacle was the name of the Op for the Mozambican bridges and was first in line. But on the 15th November 1979 during the Lancaster House Talks there was a switch and the go ahead was given to take out the Zambian bridges which effectively put ZIPRA out of the war.

As the rush was on to prepare for the Mozambique part of the op the word came through from London that the cease fire had been signed and all external ops were terminated.
In the late-1980's there was a Granada TV series 'End of Empire', with two episodes on Rhodesia (UDI & Lancaster House) and many years later the BBC had another. Dropping the bridges was mentioned IIRC in both and in one, cannot recall which now, a Mozambique government advisor to Samora Machel referred to the intense pressure applied to Mugabe to agree in London. Not sure of the dates, but that may explain why the switch of targets was made to Zambia.