The which? book doesn't make this clear and actualy says not to
connect the houseshed cable earth wire to the shed csu, connect the
shed csu to an earth rod, but what would you do with the houseshed
earth?

It seems that some people isolate the cable earth at the shed, whilst others
connect it to the earthing terminal. Obviously, there is always a connection
to the earth rod.

I can see advantages to both. Using the cable introduces a second earth to
the system, in case one earth fails. However, using the cable could result
in current flowing from the house to the shed under earth fault conditions.

However, I suspect any such flow would be shortlived and limited in current,
preventing the small earth SWA earth overheating. Also, the supplier's earth
is probably much better anyway.

In any case, there should be an RCD in the shed covering the entire
installation. The "best" solution is a time delayed 100mA RCD with 30mA RCBO
for sockets and 6A MCB for lights. However, unless you are using dangerous
machinery in the shed, this is overkill and you could get away with a 30mA
"whole shed" installation. The difference is (apart from being around 40
quid cheaper) that when you chop the power cord, the lights go out (which
isn't a good idea if you are holding a still rotating dangerous piece of
machinery!)

Christian.