Modern ones don't need much doing to them, the fitter can test the flue gases to see if it is burning cleanly and tell if there is a major problem without taking it apart. Also of course you will describe any faults or unusual events that you have noticed. He will inpect it for signs of leaking flue gases, which can be very dangerous, but are more likely on very old, or badly-fitted, boilers.
If you have a system filter on it you should empty out any dirt yourself at least once a year.
Many manufacturers offer a 5-year guarantee now, which is conditional on annual servicing.
Definitely once a year, otherwise your warranty will be invalidated. There's some boiler installers now doing 12 year warranties, and these are conditional on an annual service - with no exceptions. A service should be around £75 or free if you're on a maintenance plan. Watch out for some companies - I won't name names - let's call them Gritish Bas, who charge up to £700 for a flush of the system. Even if you've got a maintenance plan.
From a safety point of view, it makes a lot of sense too, to make sure there's no gases escaping.