Gas usage advice required

BooJewels

I'd concur with others about the thermostat temperature - 21C sounds pretty warm to me - I like to be warm in the house, but would be flushed and lathered at that.  I use about the same as you, in an old spread out stone cottage which is badly insulated, but can't be inexpensively improved - but I've draft excluders, thermal curtains etc) , but I have the heating on all day - my thermostat is 18C in the first part of the morning, up to 19C for the rest of the day.  I'm perfectly comfortable in a t-shirt and short sleeved shirt, because I like bare arms when I work - don't like cuffs in the way.

When your thermostat is at a higher temperature, the boiler has to work for more of the time to maintain that temp - therefore burning up more gas.  When you lower it, the boiler is off for more of the time as it's not needed.  I had a new boiler before winter and the biggest improvement for me has been having an efficient thermostat (and with Hive I can operate and monitor it via my phone) and it's subtle enough to only come on periodically to boost the temperature.  I know I've used less gas with this boiler and I am aware that it's off as much as it's on.  My old boiler ran most of the time - and I think most of this is to do with the thermostat working much better.