All the services are supposed to be bonded as close as possible to the point of entry.
It seems to me both the incoming gas and water supply would earth an electrical issue.
On a TN-C-S supply, your electrical "earth" is provided by the electrode at the substation via the neutral conductor in the service cable, you can't necessarily rely on gas & water pipes to do the job, because they're commonly plastic.
The problem with a TN-C-S system arises if and when the neutral gets broken, because then all the "earthed" metalwork in the electrical system can become live. The purpose of bonding lies in its full name,
equipotential bonding: it's to make sure that no potential can exist between one lot of exposed metal and another. If one piece of exposed metal becomes live you can't get electrocuted if every other bit of metal you can find to touch is also live as well:
Twice I've been picked up on my bonding, once by the boiler man from Worcester Bosch, and again recently by Cadent, I think they're obliged to tell you so that it's your fault if you do nothing about it, not theirs. The problem is that mine was installed back in the days when the point of entry was next to the gas meter in the cupboard under the stairs, and now the point of entry could be construed as being in the porch next to the front door. It depends on whether you view the underfloor space from porch to cupboard as outside the point of entry, or inside, as Cadent recently said. If anyone makes a fuss, I could point out that it was British Gas who moved the meter out of the cupboard, so if the bonding needed moving as well, they should have done it at the time. After all, if British Gas hadn't bodged the job of moving the meter from cupboard to porch in the first place, Cadent wouldn't have had to come and sort it all out last June.
