Advice wanted on boilers please

MrsConbo

We've just moved into a new home and surprise surprise the boiler isn't working well. We've thought about having it fixed but as it's now leaking water and the timer isn't working properly and it's about 15 years old we think we will just get a new one.
So we've had 3 people out to quote.
1. Independent - recommended a Baxi Duo tec 28. Quoted £1500.

2. British Gas - said we need to move the boiler and have a new flue. Quote came in just over £3000! (Nice payment plan included) and they recommended a Worcester.

3. Independent - recommended a Baxi Platinum 28. No quote back just yet. Says the flue doesn't need moving.

Question.

Is the flue placement measured from an opening window or from the main window (300mm). Ours is about 150mm from the window itself but the opening side is the other side of the window so would be well over the 300mm?

Also what are the differences between the two baxi boilers except the warranty (7 v 10yr).

Thanks