What is the normal household gas consumption per annum?

Hard to say. The size and type of boiler play a role, as does the temperature you choose to set your boiler at. You said 22, I assume Celsius. But the frequwency in which it turns on is dictated by the climate outside. If you have a warmer winter, you will use less fuel.

An older boiler will almost always use more. Even if it is perfectly maintained. Things like "pilot lights" waste fuel. Many newer boilers do not have perpetually lit pilots.

If it is not well maintained, it will use much more fuel.Mineral deposits left from years of unfiltered water, can insulate the boiler, making it take longer to heat the water.

Dirty jets can burn improperly, using much more fuel to achieve the same-size flame.

I am not sure how to measure in KwH. I thought that was only how they measure electricity.

Unless you are talking about how much electricity it uses. Not too much. The circulating pumps run on electricity, but many systems don't even have these pumps. And when they do, they don't use that much electricity.

Are you thinking of converting to electric? Anything new will be more cost efficient than anything old. But IDK the cost of electricity in Germany. In the States, it would be foolish to have electric heat, unless you never use it (Florida) or you have your own electricity source.

nothing is normal - everyone uses different amounts