It is vital your gas boiler is serviced annually. Depend of the manufacturer and the model different requirements are necessary for each model. As some of them are very common for each model other are different regarding the time frame. 

The primary objective on the big strip down boiler service is to extend the life of the boiler and calibrate the efficiency and combustion performance as per manufacturer instructions.

The most common mistake or misunderstanding on many customers is to make the difference between usual annual service and the big strip down service. Some of the manufacturers are requiring big strip down service on annually basis like other every 5 years. But on the end the best judgement should be provided by the gas safe engineer onsite, where depend of the boiler condition you will receive the recommendations and advise.