Incidentally, how would I earth bond the new stainless sink and
associated pipework? Incoming water main is plastic.

You don't need supplementary bonding. Supplementary bonding in a kitchen may
reduce safety under certain conditions.

The pipework in the house should be main bonded on entry to the building,
but only if it is metal. This requires a thick unbroken cable from the entry
point to the main earthing terminal.

Christian.