in this case if the metal pipework in the house has been de-earthed by the plastic pipe then it's no longer extraneous ie no longer needs bonding. but the op needs to confirm.
Hi Ben Been reading GN8. I concede. It also raises some interesting points. I would say that a large number of domestic properties that have plastic water mains and plastic gas mains need not be main equipotential bonded as they do not fall within the definition of an extraneous-conductive-part. It is quite a complicated subject when you read in to it but to truly decide whether the case is extraneous you have to do a test and plug figures in to a formula. I accept that bonding pipework insulated from earth is introducing a hazard whereas it would be best left alone. Given that there is a whole mess of bonding and non-bonding in the world it probably makes no difference really. Like me, got it wrong, you would have a situation where say I saw a plastic supply pipe and told the customer it should not be bonded. So I don't. Then another spark comes in and tells the customer it needs to be bonded, so it is. And that sums it up really. Nothing is ever done correctly.
Buy earthrod and clamp from screwfix bang rod into ground connect clamp, end of problem. Why all the ifs & buts, just earth the property and leave it at that.