From my poor memory: There is no specific requirement unless a outlet socket is near to the sink/draining board. If the impedance measured between the earth of the socket and the bonded earth is larger than 50Kohms then cross bonding is not necessary.
IEE OSG page 26. "There is no specific requirement to bond kitchen pipe,
> sinks or draining boards."
>
> Even if there was a requirement 4mm would probably be OK as used in a > bathroom.