Pause for thought indeed, been an electrician for nighon 30 years and
would not consider earth rod for outbuildings unless their was an
existing tt system, just as likely to loose your earth rod connection as
your main supply earth surly, how often should your earth rod be tested
3mts, 6mths a year? That just will not happen in a domestic house, As
long as rcd protection is used,imo using house earth is the safest way.

...and of course think about what happens to the earth rod impedance when we get a long hot spell &
the ground dries out...