I wouldn't replace it just because of the issues you have. They may be unrelated to the boiler itself. You need an experienced heating engineer to properly diagnose what is causing the issue. If you go to the expense of replacing it and the new boiler behaves the same way, you will have wasted time and money.
You might be better with a self employed person as opposed to BG. I always think it better to have someone working for you who knows you, your issues and your stuff, whether it be a boiler, car or something else. Having different engineers look at the same thing can mean you go round in circles, as they will all be starting from scratch.