Thanks everyone for the answers. I suppose the MetOffice handbook is a reasonable source for the CAA to use, even if the terminology may be outdated.
Unfortunately, that means that I will have to start teaching that warm sector depressions have circular isobars (which is a deciding factor in the 'correct' answer in the new exams). From all the information I can find, it would seem that the cold sector has circular isobars and the warm sector has straight, parallel isobars.