I've just heard back from Anycubic - they're sending out a replacement rod and 2 brackets: my left rod seemed fine and moved freely within the brackets, but right rod wouldn't move at all - stuck tight! I'm not expecting it to be a miracle cure and have resigned myself to not ever printing a full 200x200 object. At present I can get about 140x80 reliably printing if I use a raft. so will hopefully improve on that.
On a side-note, I've got an infrared measurement gun now and found a 25mm strip on the left from front to back sits at 30-40ºC when the rest of the bed is set to 68-70º.