I can confirm that some of the wiring goes back to 1975 and I have come across cables with a CPC that looks thinner than todays standard, so that might explain the discrepancy in the readings.

To do the installation I knocked out three of the six rear knockouts, two are now quite stuffed with cables the third has still some scope and includes a 6mm2 T&E for our existing shower which will become defunct once we do the bathroom and move over to a digital shower. I still have to get at least 3 × 2.5mm2 cables in and taking a good look at it I should be able to get those cables plus the earth bonds through it. I know the easy answer is to use another knockout but I am trying minimise how much of the CU integrity is compromised making another 40mm diam. hole just for 3 cables seems OTT. All the bonding on the old plastic CU was done on the internal terminal strip so will just be repeating what was already correct.