It's probably the pilot light. That's an old boiler with an efficiency of maybe 65%. A modern condensing boiler would manage more like 90%.
If you've not got a manual you should be able to find it online.
https://www.freeboilermanuals.com/assets/pdf/glowworm/7spacesaver-30B-MkII-installation-manual.pdf
Eruru
June 2022