Disregarding the efficiency of the chargers:

42 amps x 12 volts = 504 Watts

504 watts / 240 volts = 2.1 amps

-=BUT=-

You aren't going to be charging at 12 volts, you are going to have to take your 12 volt batteries up to something more like 14.4 volts to completely replenish their charge, so:

42 amps x 14 volts = 588 watts

588 watts / 240 volts = 2.45 amps

Of course, the more completely charged the batteries get, the lower the charging current will be, so using 588 watts would be considered "worst case" power draw from the AC mains.

Now, if you knew the efficiency of the chargers, you could multiply the wattage by that factor, and come up with a real-world value for your AC supply.

As for the speed of electricicty:

A lot of things can affect the speed of electricity, the type of conductor, the temperature, the frequency (if AC current), etc. but the commonly accepted "rule of thumb" seems to be about 2/3 the speed of light. Since the actual delivery of power is accomplished by the electrons knocking around inside the conductors, the real-world speed of the electrons moving through the wire is something really slow like .1mm/second.

One of the example I read was: "If you had a piece of pipe filled with tennis balls and you stuffed one too many into one end, how fast would one be pushed out the other end? How far did the ball you put in the tube move? If the tube was 100 miles long (ignoring friction), how fast were the balls moving to deliver one to the opposite end?"

Electric power delivery is similar, and especially so in AC systems. The electrons that the power company is disturbing at their end will never actually arrive at your mains receptacle, they will only cause "pressure" that moves the electrons in the wires at your end.

Confusing enough?