Micheal, I obviously don’t know the exact details of your bungalow, but in general they have a much greater external surface area to internal space compared with a two storey house.
My first checks would be on quality an efficiency of wall and roof insulation ie does your bungalow have cavity wall insulation? Is the roof space of adequate depth, this is easily fixed.
If the walls aren’t insulated that would be my first priority, either cavity fill or install external or internal insulation.
Secondly, if you’re windows have been installed for some years they may not be as efficient as modern double glazing ie breaking down of seals etc, you could always enquire about triple glazing.
My final point would be to look at installing air conditioning which is also used to provide air sourced heating I’m sure it would be cheaper to run than an oil fired boiler.
Andy