Best answer by Transparent
Your first Forum posting I notice!
Although 14 years may sound a long time ago, I would still expect the boiler to be rated SEDBUK "A". Type the model number into a search engine to confirm this.
"A"-rated can only be achieved if it is a condensing boiler with a condensate drain leading outside or to a waste pipe. It is certified to operate up to 95% efficiency under a predetermined set of operating conditions.
I also happen to know that Alpha boilers of about that age could have an optional GasSaver heat-recovery unit mounted on top to remove even more of the energy which would otherwise be wasted through the flue. Does yours have one?
Next, are you making a fair comparison with the boiler in your old house? The main issue is whether each of them was a Combi style.
Even if you understand this, for the sake of others reading this topic in future, can I define Combi as a boiler which not only provides heating (radiators) but also a direct supply of Domestic Hot Water (DHW) when you turn on a hot tap. A Combi boiler does not have a hot-water cylinder which it maintains at a preset temperature. Instead it fires up a few seconds after water flows from the hot tap.
Once you give me that background info, I'll be in a better position to advise where you are losing efficiency, resulting in higher than expected gas bills.
Oh... and please fill out your user-profile. I read this to better refine my answers on the Forum. Thanks.