What is the point of having an earth rod if you have a fault and the
current
flows through you to earth, surley the earth rod does not do anything ??

The earth rod is connected to any main equipotential bonding (in your case
none) and the earth pins of the sockets and all the protective conductiors
in the circuits.

What is the point of doing an earth loop test if there is an RCD protecting
the garage??

You must ensure the earth loop impedence is low enough so that the RCD will
trip before the earth potential reaches 50V. This requires an earth loop
impedence below a certain value (around 1600 ohms). However, I believe the
recommendation is to have one below 200 ohms, which is well within the 50V
requirement when using a 30mA RCD. (It will drop 6V by the time it reaches
guaranteed trip current). It allows for changes in ground resistance and is
very easily achievable, so accepting less is asking for trouble.

so I could test from a live wire in the fuse box to the earth rod for
example and get a reading??

Yes. The testing will attempt to draw a predetermined current (usually 20A).
It then measures the voltage drop. If the earth loop impedence is high, the
voltage drop will be greater. If you are working in low current circuits,
such as lighting, it might blow the MCB, though. Lower current testers are
available for this reason, but you should use the full current to test when
possible.

Christian.