Mains installation in garden

["rb_ziggy" <rb.lists@xxxxxxxxxxxx>]

I'm looking for a bit of initial help on feasibility of installation
before I call in a sparky.

I'm looking to put in a series of 12V lights (fully automated of
course)into the garden but the cable runs look to be 12-15m which
seems to be above the run distances where voltage drop off starts to
be an issue.  A couple will be 50W spots and these seem to cause a
prob - 8m seems to be the recomended max run.  (Originally I'd
planned for transformers to be mounted in a weatherproof enclosure
attached to the house but the scheme is growing!)

So I'll need to take mains out to a transformer in the garden.

I have an external socket nearby (RCD protected but probably on one
of the regular downstairs ring mains).  Putting in a new spur from
the consumer unit is goint to be practically impossible.

So:

1) Can I extend (have extended) this external socket point further
into the garden using armoured cable?  (Or does it have to be on an
entirely separate circuit from the consumer unit?)
2) What depth does this need to be buried.  (We have laid a conduit
about 3" under hogging - originally intended only for LV purposes.
So with proper armoured cable would this be ok? I seem to remember
that any mains should be 18" deep)  If I have to trench to 18"
it's
likely to be a non-starter now!

Any other good ideas, e.g. experiences for running LV over longer
distances?

This work will be done by a proper sparky but I'd appreciate help
before I waste time on asking for a quote that is not practical.

Thanks

Richard