No worries mate,

You've done the calculation fine, its most likely a different calorific factor used in the calculations on your bill. It should show you somewhere the number they have used. (39.2 would make 1701.71kWh)

If you really want to find out more about the calorific factor and what it is, you can do so here, its a little technical though..

http://www.nationalgrid.com/uk/Gas/Data/misc/reports/description/

As a point of reference for your usage, the accepted national average for annual gas usage is 20,500kWh. Based on your October monthly usage of 1701 x 12 = 20,412kWh. (This average is based on a three bed house I believe)

Its your prices that are the problem, those Spark prices aren't very attractive, particularly without the direct debit discount. My best advice would be (and i know this sounds crazy) set up a direct debit with them for ongoing usage only while you dispute the balance. I know this sounds mad but you should take the cheapest price you can from them until this is sorted and you can switch. (they will be unlikely to allow you to switch if you have an outstanding balance, something to be aware of)

Make sure they keep the debt aside while you are disputing it, and let you pay a fixed amount monthly. It'll save you time and money in the long run.

Let me know how you get on..

G