I noticed that the main water inlet to my house, as it enters a 1970s
brass stop cock and drain tap assembly is weeping slightly. I can get
a reasonable grip on the nut with a wrench and support the stopcock,
and the pipe below it is embedded in grout then concrete about an inch
from the stopcock.

How much brute force and ignorance would it be wise to apply when
tightening up this nut? What other approaches are there?

I have easy access to an external stopcock upstream of the pipe. The
obvious failure mode is to break the pipe and/or stopcock assembly, in
which case if I was doing it under pressure I'd expect a reasonable
amount of spillage which shouldn't be too much of a disaster.

Thanks,

Dickon