The earthling conductor is connected to the earth bar in the CU as it
should be. Should the outer steel armour be earthed?

The armour should be earthed at the house end.

There is no need for it to be connected at the garage end, provided some
other form of earth is used, such as an earth exported as a conductor within
the SWA (which appears to be the case here) or an earth rod.

Christian.