here is the answer I came up with, and explanations to my method for following, this can be used to find the heating for room heaters also.
work our the heat input to the water knowing that 4.2kj is needed to heat 1 litre of water 1 degree Celsius.
So 15 l\min is  900 l\hr helps for this
  900 l\hr x 4.2kj x 28 degree temp rise =105840 kj\hr.      can be changed to 105.84 Mj\h
the input to the appliance, use the efficiency = output/input, so change the equation to suit.
Input to the appliance is        105.84 M\hr / 0.75 = 141.12Mj\h.
the formula that I used to calculate gas through a meter is : m2 = mj\h x cv x 3600 / time(sec) if you like to know.
gas rate is          141.12mj\h / 40 =3.528 m3\h

Hope this is useful and correct. I would be interested also if anyone has another method so I can be enlighten too