Doing an EICR today and two ring finals dead tests indicated that the CPC was exactly half the CSA of the line and neutral. If it had been old cable I wouldn't be surprised. But one was wired with brown/blue and the other relatively new looking red/black, They should have been 2.5/1.5. When I measured the cable with a micrometer the cpc worked out at a csa of about 1.2mm and the main conductors 2.5mm.
Not an issue from a safety point of view as it was on a TT earth with low fault currents, but then I started to think what is the allowed minimum and maximum diameter/csa of the cores according to british standards and I haven't been able to find any data, hope others can point me in the right direction.