Where pipework in a bathroom is plastic, do I need to supplementary bond the bath/radiator, etc?
No; if the pipework is substantially plastic with only short metal stubs connecting the taps/radiators, etc. these can be treated as isolated pieces of metalwork and do not need to be bonded. On metal pipework installations, under the 17th Edition, provided all the circuits of the location are protected by a 30mA RCD and there is continuity between the extraneous conductive parts and the protective (main) equipotential bonding, then supplementary bonding is not required.