OSG 2008 (red book) 4.7 Supplementary Bonding of plastic pipe installaytions
Supplementary bonding is not required to metallic parts supplied by plastic pipes.
Table 4.2 note v "If incoming gas and water services are of plastic, main bonding connections are to be made to metal installation pipes only, where required. "
4.4 Main protective bonding of plastic services
There is no requirement to main bond an incoming service where the incoming service pipe and the pipework within the installation are both of plastic. Where there is a plastic incoming service and a metal installation within the premises, main bonding is recommended unless it has been confirmed that any metal pipework within the building is not introducing earth potential. All bonding connections connections are to be applied to the consumers side of any meter, main stopcock or incoming insert.
He is just trying it on and trying to get the cost of the boiler from you, check your initial letter as you stated it was just installing and fitting the boiler, unless you stated that you would commisssion it as well then I don't see what he can do.