I have not dealt with the green scheme so cannot comment on but generally any discount has got to be good news. Just be careful that the supplier doesn't mark up the boiler to give THEM the benefit. You can double check that by getting comparable quotes. At 15 yrs old your boiler has done well (10 years is about average) but will not be working efficiently and your fuel bills will be higher as a result. You could save 20% or more on your fuel bills with a new boiler, which needs to be taken into account when considering the cost. (Although that doesn't make the upfront cost any the less frightening.)

Do you have any kind of service plan with say British Gas? Because if you do you might get a discount that way too? Generally though I think you will find that a good local plumber and heating company will do as good if not better a deal. It's what we did a couple of years ago.