While digging the garden I came across a 8mm armoured cable, about 8
inches under an old lawn with yellow plastic warning buried about an
inch over the top of it. It appears to be running towards a street
lamp.

I didn't damage it or disturb it much, and it isn't in my way.

Is a 8mm dia armoured cable - diameter across the steel armour - about
right for a small street lamp? It seems to be running from the school
next door. Should the council be paying me a rent on this bit of space?

I thought that buried cable should be at least 18 inches down.

R.