What exactly is a Boiler Service?

I have my gas combi boiler serviced every year by my landlord but I am a bit concerned about how it is being serviced.

What is a service? The reason I ask this question is because I remember seeing the engineer who first serviced my boiler removing the front cover and doing checks and also cleaning out the burner etc.

However a new company has taken over the servicing in the last 5 years and they have never yet removed the front cover, they just look at it and thats it, is this right? On the sheet it states that they have done a safety check and a service?

I am not qualified in anyway in regard to servicing gas combi boilers but like a car don't you have to lift the bonnet (boiler cover) to service it?