Hmm. I'd convert 12kW gross flow by dividing by 10.76, giving 1.12 m^3/hr, which everybody seems to be happy with. According to Table 1, here:
https://copperplumbing.org.uk/sites...achments/domestic-gas-planning-and-sizing.pdf
1.1 m^3 / hr gives a pressure drop of 1 mbar along 15m of 15mm/0.7mm tube, i.e. a drop of 0.067 mbar/m, which is very close to SaunCorbs's value in post #9.
However, if one uses the more sophisticated formula and calculator here:
Natural Gas - Pipe Sizing Calculator
with 13.6mm (id) for the pipe, 1.063 mmH2O (0.1042 mbar) for the pressure drop, and 1m for the pipe length, and 0.6 for the specific gravity of the gas you get a flow rate of 1.13 m^3 / hr, which is close enough to make me think that it is this formula that has been used to set the test and calculate the model answer.
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