i think you can use some large sized allen keys to loosen (one in the top and one in the bottom)
if it isn't then try wraping a towel round one of the halves and using a set of pliars or a monkey wrench to get more grip and leverage (but be very carefull not to scratch th anodising off)
Get yerself a couple of small strap wrenches, a few quid from the DIY or hardware store. Or you could get someone with a strong grip , use some latex gloves.
Unscrew the adjuster from the bottom of the reg, then unscrew apart both top and bottom reg housing. Becareful not to lose the bits when you take them apart.
Torpedo regs have a limited lifespan. The internals gradually score a groove into the body until it stops working. (may also explain why you cant open it)
Typical Bob Long really. Too cheap to even grind the springs flat like they were supposed to...