Is there a "main earth bond" in such a system? If so where is it?
I'm concerned only because if there is a "main earth bond" then should
it be some proportion of the meter tail size (even on a TT system)?

TT doesn't need the large earth conductor sizes that TN-C-S (PME) requires,
as the RCD ensures that any earth current flow is short lived and requires
only 100mA to trip, so earth loop impedence is not particularly relevent
(within reason).

However, it is a VERY good idea to install the TN-C-S recommended sizes, as
it will enable you to upgrade to TN-C-S when it becomes available in your
area.

Christian.