I've been looking at a pal's installation where the CU really needs updating due to extras having been added over the years. The house is '60s built with imperial wiring in PVC, but earths appear to be present everywhere including lighting circuits. The CU *I think* isn't original as it has MCBs and much appearance of 'bodging' inside - lots of chocolate blocks. The house has been altered quite a bit and the wiring additions look like standard builders work...;-)
However, the main problem I can see is the safety bonding. It's a PME supply, in the built in garage where the CU is also. There's an earth block, but only the gas pipework is bonded - nothing to the water system. And that's the snag. The stopcock is in the kitchen and an extension has been built between it and the garage making concealing an internal earth wire virtually impossible. However, there's a mains water tap in the garage which is fed off the adjacent cloakroom which is all plumbed in copper and there is continuity from that to the pipework in the kitchen after the stopcock. So is it acceptable to bond to that or will it need an earth run all the way round the outside of the house to the kitchen?