To answer your questions:
The armour cable does already exist. It's buried 2ft in the ground, its about 10 metres long, 3 core, 2.5mm2 (http://www.screwfix.com/app/sfd/cat/pro.jsp?ts=29813&id=19322)
I don't know the load that it will power as I haven't got the water feature (but will...