The descriptions in the Bristol GS, CATS, and PadPilot ATPL books all either depict or state the warm sector isobars are straight, but a handbook written in the 70s and last updated in the 90s depicts something different. Really... who cares? We're pilots, not meteorologists. Before these questions appeared out of nowhere in the new and improved exams (at PPL level!), I had absolutely no interest whatsoever in what shape the isobars are... nor would any PPL holder find this information practical.
I suppose if I want a definitive answer as to what it is the CAA has decided I should already be aware of and be teaching, I'll have to ask them. The references to textbook pages in the old answer booklet were useful.