Is a neutral back to main feed from satellite circuit box necessary if neutral is bonded to ground remotely?
It sounds like any current taken by an appliance in the shed is traveling thru the live #6 wire and returns via the soil back to the main box at your home. This is unacceptable in the UK according to wiring regulations. 

The reason it would be unsatisfactory is because earth (and earth wires) are reserved for enabling the detection of faults (should one occur). Imagine you have a hand operated drill that developed a live wire fault to the metal on the drill - you'd be electrocuted and there would be nothing to protect you - any current passing through you to earth is undetectable from actual appliance current.

Many, many places insist on RCDs (residual current devices) - these wire in series with live and neutral and any current that does not flow through both live and neutral, generates an imbalance current that can be detected and trip the supply. This usually means you will receive up to 30mA for a few tens of milli seconds maximum - short enough to prevent you frying.

Another problem of not having a neutral and relying on earth return thru soil is that if the soil dries you get no decent connectivity.

If you want to avoid this do some math on what #6 wire can safely withstand and rewire one of the two wires making it a neutral wire.