As I said, I suspect that the reference to 'earth bonding' may well be a red herring - or, at least, mis-described/misleading.
Indeed, it is not clear as to whether the reference to "earth bonding of all the metal equipment" really dies relate to 'main bonding' ('PEB') - it could be just talking about the earthing (to MET) of 'all the metal equipment' (which is what have been required to facilitate operation of an RCD, had one been present).
I would imagine that its very unlikely that the wiring of the 'heating element' concerned did not result in the surrounding metalwork being earthed.
Kind Regards, John