robgraham wrote:

Two questions - as the TT earth is relatively high resistance compared
to the supplier earth, and hence the fault current will not be high,
does the earth cable size have to be any bigger than 2.5mm ? If it is
to be bigger, what is the justification for this?

By "earth cable" do you mean the earthing conductor - i.e. the
connection to the earth electrode? If buried for part of its length
this needs to be 16 mm^2 on grounds of mechanical robustness. If not
buried then 4 mm^2 is OK. In both cases the size can be reduced to 2.5
mm^2 if the conductor is mechanically protected by being run in conduit,
etc.

Secondly, although there is no water pipes, etc, the garage roof is
steel frames; should these be bonded ?

There is no need to bond structural steelwork unless it's in contact
with the ground.

--
Andy