If this is the case Namsag is correct , my understanding is that a shuttered socket ( one with aswitch on) only isolates the live feed to appliance not the neutral ??? if the switch is used , as opposed to pulling the plug out ???
Correct, if the plugs polarity is revered then the neutral will be live and just switching it off will still leave the appliance live.
A double pole fused spur is best method despite agiles take on it, as a double pole isolator disconnects completely.
Regardless of method of isolation basic dead tests should still be carried out before sticking fingers in boilers. Some boilers may have 2 sources of power, some kestons do