So now I know that it takes 1.15m^3 of gas, for which I pay 50p, to heat a languorous warm bath . Possibly I need to get a better deal from my gas company.
Time for some nostalgic Elementary Chemistry. Natural gas is basically methane, molecular weight 16; I hope I can assume the cubic metres I'm charged for are of 1-atmosphere gas, so 0.024m^3 per mole, so 1.15m^3 is 48 moles, or about 750 grams.
Methane yields 900kJ per mole when burned, so 44 megajoules, which is enough to heat 100kg of water by, ah, 4.2kJ/kg/C, about 100C ... since I am not more thoroughly boiled than a lobster, I feel there's an inefficiency somewhere. I've no idea how much water there is in a languorous bath; my water is unmetered and my curiosity doesn't extend to filling the bath using a measuring-jug, and besides I tend to run some water out and run hot water in as the bath cools.
I've contributed 48 moles of CO2 at 44 grams per mole to the atmosphere, or rather over two kilos; offsetting this at a rate of £9 a ton costs about 2p per bath or three pounds a year, which makes me wonder why the gas company doesn't just offset all its CO2 emissions, produce an environmentally sound press release on recycled paper, and raise prices 5% to more-than-cover it.
 I cannot help thinking that I've misspelled that, and sung the praises of a warm bath full of large grey aggressive monkeys of the sort that haunt Indian railway stations. More, or less, fun than a barrelful of monkeys?