Teapots:
"Lets say a 24kw will struggle in a 3 bedroom property, more struggle the more gas it will use."
I don't really agree with that. A boiler will need to provide heat by burning gas at a sufficient rate to satisfy the demand made by the house (which varies with size, air-change, insulation, temperature required by the user etc). If 24kw is insufficient, then it won't heat the house properly, but it won't use more energy than a, say, 30kw boiler which isn't struggling.
The new boiler should have been checked for efficient setup, and should be more efficient than the old one. The problem with the OP is possibly due to colder weather, or the extra radiator. It would be interesting to know what meter readings were used to assess the before and after situations.