Not sure what that testing means? Firstly it's not winter so your outside Vs inside is like a 8 degree difference. Secondly you went from some temperature, let's say 21c indoors at this time up to 25, which uses more energy than maintaining 25 there after. It would be better to look at your base gas use in summer when the heating is off (hot water and cooking use) Multiple that by 12 and then subtract that from an annual bill. That'll give you how many kWh per year you use for heating. Then average that number by how many days your heating is used to get a kWh per day usage over a winter period. It won't be accurate on a per day basis as you'd expect to use less in a day in Oct Vs Jan for instance but it'll give you an idea of a average daily cost. For my previous patterns for instance I can expect it'll average to around £5 a day (53kwh per day) over the winter period of Oct to March (18 hours a day heating at 21c with a 20 year old non condensing boiler)