I can put only a couple of numbers for gas leakage from distribution networks. A first Boston sampling study a few years ago claimed 7% leakage and a more recent one appears to have revised this to 1-2% iirc.
I've tried to get the figures for British Gas but even though I worked in the same building where gas suppliers and DECC reps meet they're only concerned about theft and swathes of incorrect or missing meters.
What's a sensible equation for what you're suggesting?
methane warming = total warming - fossil fuel CO2 - organic CO2 +/- ?