My own personal notion would be as I say below, running mains cable -
probably 4 core, 10mm SWA, at a depth of about 3 feet, backfilling
about a foot,

10mm! - thats capable of about 60A!

And why 4 core?

I was thinking 2.5mm (29A) 2 core

then running co-ax in a conduit, backfilling about nine
more inches and then running alarm and CAT5 in conduit.

You don't think the Coax, alarm, and network will be happy together then?
Is this because of the network - I could use STP?

Sparks...