JJJ wrote:

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

Kitchen sinks do not require 'earth bonding'. Supplementary bonding of
such extraneous-conductive-parts is only necessary in bath and shower rooms.

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,

It's OK.

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)

That's not necessary, but make sure that the main bonding from the main
earth terminal to the incoming gas and water service pipes (& oil, if
applicable) is in place, close to the point of entry of each service.
The minimum size for this is 10 mm^2.

--
Andy