Well said paul . Amazing how many people actually believe this 35% gas saving.
The saving in using a condensing boiler in full condensing mode is 12% or less.
BUT because a condensing boiler has lower flue temperatures ( 50° compared with 130°+ ) it is always more efficient than a non condensing.
This figure of 35% ( actually quoted as "up to 35% ) in in some British Gas sales material and probably others as well. But that is based on "worst case" of an old back boiler or a massive cast iron boiler and not a modern copper heat exchanger boiler like virtually all combi boilers.
The saving from a non condensing combi to a condensing combi would be about 15%, not massive but still useful. However, its not going to give a payback by just changing to a condensing boiler unless there are other reasons to change like end of life or expensive breakdown.
All modern condensing boilers should last at least 10 years. There are many old Glow Worm condensing boilers ( Ulitmate ? ) which are now about 15 years old and still work for many years without servicing.
Tony