The shower itself will be earthed: it certainly doesn't rely on its
water supply pipe for this in any way.

The shower will also need supplementary bonding to other metalwork or
electrical appliances within the room. The bonding cable can be attached to
a special terminal for this purpose, or the circuit protective conductor
earthing terminal.

(Q for the panel: if the incoming water pipe is MDPE and the
installation pipework is also plastic does one x-bond the stopcock?)

No. Only metal services need to be main bonded. Taps, including stopcocks,
do not require main or supplementary bonding. For example, it is not
necessary to supplementary bond a bath tap if supplied by plastic pipe. If
it is supplied by a metallic pipe, then this supply pipe should be
supplementary bonded, though.

Christian.