Was going to say the same thing!Sorry if I have missed somthing....but...
Your boiler is 13 years old, which is not that old and it sounds like it is working fine.
Why spend a lot of money, just to save a smallish amount of money per year. It will probably take you 5-10 years just to make the money back in savings. Probaly nearer 10 years if your current boiler has an efficiency rating in the high 70s.
Maybe spend the money on better heating controls, thermostatic valves, insulation. All these other things may have a much bigger impact and shorter payback time.
I would only consider replacing a perfectly functioning boiler if it is beginning to require lot of uneconomical repair, it has actually failed or you are doing a major rennovation and would need to replumb it anyway.
Going from nearly 80% efficiency to 90% is going to say you naff all! All parts are still available for the 24i too.