So as the earth for the garage is using the SWA of the cable from the house
this is surley much better than the earth rod in the mud which I imagine
must alter its resistance wildly between the very wet times and the very
dry
times of the year if a regular RCD test proves ok surley this is good????

The earth would certainly be better in terms of loop impedence. However, the
problem arises because the earth potential of it could be different from the
earth potential of the physical garage ground.

Christian.