But if hcf stands for hundred cubic feet as it probably does, then you've actually used 30,000 cu.ft. Put that into the calculator I linked to in the beginning gives 8792 kWh, not exactly the same as the figure of 9502 given on the EON bill, but not a million miles away, and probably accounted for by different values of the other factors being used (my calculation doesn't say what it assumes those are). It also explains why they use 2.83 as a conversion from cu.ft to cu.m as it takes into account the apparently missing factor of 100 mentioned earlier (300 x 2.83 = 30,000 x 0.0283).
In which case the calculation looks OK to me. Whether ~9000 kWh of gas is a lot over approximately the three winter months, I can't comment.