The female from the 1954 marriage is shown once in electoral registers 2002 -2010. Unfortunately, the record is not refined to a year. She is living in the same area as the other records already mentioned.
You could search for a death for her, otherwise she should not be named as it is possible that she could be living.
As you know, Tony Sidwell/ Brett died in 1987 and his will might indicate if the couple were together at the time of his death.