I've just discovered that the kitchen sink in the house I've just moved into
has no earth bonding (plastic hot & cold supplies).

I'm going to fit a waste-disposal unit, so that would provide an earth
through it's metalwork to the sink.

Is this enough, or should I run earth cable from the sink back to the
consumer unit (& if so, how thick would it need to be - about 25' from the
sink to the CU)

thanks,

Jim