Can this leak have sealed itself and should I leave it alone?

[Rickey]

Hi
When my boiler was serviced this year the plumber found the leak as shown in the photo. I was told that the problem may have resolved itself but if not I may need a new boiler. I have three questions please:
1) What has caused this problem?
2) Now I know the problem is there (it's normally boxed in) I will keep an eye on it, if it isn't currently leaking should I just leave it alone and monitor or do something and if so what?
3) If it is still leaking why would I need a new boiler and not just replacment pipe/joints?
Many thanks
Rickey

 

[ShaunCorbs]

I would get the joint replaced it could require a new boiler if the leak rusts a hole through the boiler casing / flue replaced