Even if you use natural gas, butane, propane, oil or diesel boilers, water heaters or central heating, it’s still a good recommendation to have it checked every year.
My tip here is to find a reliable local installer for whichever system you use, to perform a yearly check.
We recently had our central heating caldera inspected and serviced and it cost about €80. This included flushing and cleaning the system, along with replacement parts (due to calcification, we run off well water). Can’t grumble at all at that. The previous owners hadn’t bothered servicing the system in the five years since installation, so this was likely the heaviest service cost. Next service is due, which shouldn’t cost more than €30 to €40.
As an incidental though, I am aware that some of the bottled gas companies suggest you “have to” pay an annual inspection charge. This actually isn’t the case and is entirely optional. Some of the chaps that come to inspect installations, do tend to make it sound like it isn’t optional. They’re just trying to commit people to their service. You’re entitled to use whoever you want to perform such inspections or checks.
I once helped a client organise connection of a new caldera, to run of bottled gas. The bottled gas supplier insisted that the installation wasn’t correct and one of their service engineers would correct it, whilst also insisting the client signed up to their service contract scheme. The heating engineer who had installed the system actually happened to be there. He gladly pointed out the failings in the gas bottle company inspectors “inspection”, at which point the gas bottle gas bottle guy left rather sheepishly. The installation actually exceeds regulations and the original installer has called out once each of the last two years, charging no more than €10 for his time.