In article ,
JTM writes:
Hi,

Thinking of putting in another CU. Can I take the earth to
the first CU or do I have to take it to the earth rod?

You should take it to the system's main earthing terminal.
That might be in the first CU, or it might be separate.
It's the point where all the service cross-bonding, the
earth(s) from the circuits, and the earth connection from
the supplier or earth rods (via a Voltage Operated ELCB if
fitted) are all connected together.

You need to bring the whole system up to current earthing
standards too, if it isn't already (including replacing
any Voltage Operated ELCB with RCD(s)), and test the
effectiveness of any earth rod(s).

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]