While I was ploughing through the Times Digital Archive for United Glass info last night, I found a piece which said that UG had sold Phoenix in early 1969. Not 100% clear to whom, but in May 1969, they were reported to be in negotiations with Jobling, owner of Pyrex, Phoenix's main competitor and the dominant player in the British market (80% of market share).
So my guess is that Jobling bought Phoenix and either closed the operation down completely or at least absorbed production into the Pyrex operation and killed off the Phoenix brand.
Will investigate further once I have access to the TDA again (apparently I can get remote access through Westminster Libraries).