This is why I said above, "I do about one a month".
Much of that is my choice as I enjoy bathrooms (including painting,
tiling, woodwork, etc) and also general central heating problems.
Where I live I think there's one person who seems to service boilers (oil and gas) and that's all he does. The rest of us - are cowboys [cough!!] - I mean the rest of us do general plumbing and most do gas and some do oil. I think there's a much "better" market for gas as more people have it (even in the sticks with LPG).
I felt there was plenty of competition with gas and that I'd do better with oil but the only reason I did it was I didn't want people phoning me up saying "My heating doesn't work" and me not knowing the fogiest about how a boiler worked. Although I pass my gas work to someone else (about 1 call every 2 months) and he passes me his oil stoves (about 1 call every year) I'm happy with my lot of 8 boilers to service and 2 installations a year.
The number of electrical showers I install and taps I fix and ...
You could make a good living but it won't happen for a good year or two or even five. If there's one thing I've learned it's most jobs come from recommendation and it takes half a lifetime to build up a good reputation. People also don't think that servicing an oil boiler is a high priority and unfortunately for us plumbers, oil boilers are very reliable so don't show any faults for a long time. And if they make funny noises they still keep going!!
Not really answered your question because how you market yourself could turn my negative points on their head. If you tackle the safety and economy aspect of oil boilers and keep plugging away encouraging customers to get them serviced regularly there's no reason why you shouldn't make a good living. You probably won't be a millionnaire but no reason why you can't earn enough for a few nice holidays.
Hopefully this week will be busy as the weather is going to be cold ...