Well, "insistent" maybe too strong a word. As grandparents I'd have thought it pretty natural for them to want to see the boys before their holiday. I imagine they had enough control of Sheila for her to arrange visits when they were 'invited'.
We don't know, despite that you frequently remind us of her visits to her doctor, how ill -or not- June actually was. She appeared to be a fully functioning member of society. She ran her home. She shopped. She drove. She was a church warden. She oversaw various church activities. She was a busy, busy woman, none of which equates with a depressive. Maybe it was no more than low level anxiety. Maybe she was a hypochondriac. We can speculate from here to eternity but I doubt we'll ever know when she wrote that letter.
In CAL's book we get:
"
In the car he told me he didn't like being threatened and that I scared him," Julie remembered. She picked up an envelope from the cassette compartment but he took it off her. "He told me that it was from his mother June and was to be opened in the event of her death. He told me it had been written recently and was a letter apologizing for the way his mother had sometimes treated him. He said that his mother loved him, but he said that it didn't make any difference about the way he felt about her."This tallies with Jeremy telling Julie that his parents had "forfeited the right to live" for the way he had been treated. One can only surmise that he blamed his mother for the break up of his relationship with Suzette and possibly even the miscarriages due to stress, whilst Sheila had produced two healthy young boys, who seemed the apple of June's eye. Nevill gave orders to be followed on the commencement of each working day, which Jeremy tolerated in the cannabis-filled haze of the tractor, longing for the moment when he could escape to the bright lights of London.
I think something just snapped in Jeremy as he sought a way out of his predicament. The eight wasted years at Gresham's came back to haunt him as he realized his lowly social status amongst his work colleagues, and he was determined not to grow old prematurely, which is what he discerned when he looked around his environs. Maybe he was taken for granted as a skivvy on the Farm, his only pleasure in the workplace in bending the rules with his flowery garb and cosmetics, but his portal to absconsion was evil personified. It entailed the loss of two bright, young lives, twins mature beyond their years who would have remained firm and constant friends throughout life's journey, the destruction of two people who deserved to wind down in their twilight years with their own gratifications, and a mother who desperately needed care, not the barbaric annihilation that was to become her fate.
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