When a repair can make sense
If the fault is clear, the rest of the boiler is sound and the correct part is available, a repair may be the sensible choice. I will not sell you a new boiler just because the existing one has needed a visit.
A service history and a heating system that has been looked after also make a repair easier to judge.
When replacement becomes worth discussing
Repeated faults, several worn components, poor access to parts or problems with the wider system can change the decision. So can a boiler that no longer suits the amount of hot water the house needs.
The useful comparison is not one repair bill against one new boiler. It is the likely repair, the condition of the rest of the appliance and what each option gives you from this point on.
Get both options in plain English
I will tell you what has failed, what a repair involves and whether I think it is worth doing. If replacement is the better route, I price it separately so you can decide without pressure.