What's the equation to convert gas meter readings into kWh?

Yep BPLightlog is on the money once again. 

I make 7 cubic meters of gas ~75 kWh of energy @tval, using the formula above. 

Hi @franR , that looks close to being the calculation for an imperial meter 

See the extra x2.83 for those meters. 
If yours is not imperial but metric, you will need to get that changed via customer support. 
If you’re not sure, then post a photo of the meter and someone will be able to check for you

Some of my Gas Readings are changing on subsequent days as per the example chart below … Mostly they are increased but sometimes they go down. Is this a sign that there is a problem with the meter or is it a ‘balancing up’ within OVO? The columns contain the historical readings which appear on each day. Data in Blue are the changes, if any, which appear a day or two afterwards.

Thank you … if that’s the case I’m not sure why only some of the values change … and one of them changed 2 days later … 

First, those aren’t ‘readings’. A meter reading is the number you see on the meter’s dial or display, nowadays almost always the number of cubic metres of gas that have passed through it since it was first installed. The numbers in your table are usage figures converted to amounts of energy consumed in a specific period. I’m not sure where you’re taking these figures from, but if they’re from the meter itself, these are for guidance only. The figures used for billing should be available online.

Next, BP has explained why these numbers might change, because the calorific value of the gas consumed on any particular day will not be known until a day later and sometimes a few days later. 

The calorific value varies around 40 MJ/m³, but it should always be between 38 and 41 according to the OPSS, the standards body responsible. The line in your table that shows one figure increasing from 89.79 to 97.45 is suspicious, because the values can’t both be within the prescribed limits. If 89.79 was calculated using a CV of 38, it shouldn’t be possible for it to rise above (41/38) x 89.79 = 96.88. It’s possible, though, that an older OPSS direction allowing for a maximum CV of 42 MJ/m³ is still being applied. I don’t know it that’s legal, but that’s a question for OPSS and the gas transporter concerned. OVO have to use the figure published by National Grid. 

From a quick survey of CVs for 17-20 February in my region, it doesn’t look like there has been any sort of revision of that order.