I don't think there is any electrical reason to supplementary bond a boiler which is installed inside the equipotential zone unless it is in a special location such as a bathroom etc. I believe there may be Corgi regs which say they need to be supplementary bonded, any plumbers confirm this? Main equipotential bonding (size depends on supply type) should be applied to parts which can introduce a potential different to that of the equipotential zone, i.e. metal water / gas pipes can introduce true earth potential into the zone.
