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Gabor is quite right.
It is a very nice picture of Rhagio strigosus, features:
* spotted wings
* pleurae partly yellow
* third antennal segment very short (shorter than second).

If you are looking for a key, start with Tierwelt Deutschlands (that is, if you can read german). Cheap, for instance at www.insecta.de.
I know, it's old, but for this family still a good start.

OR the British key by Stubbs (Soldier flies and allies).
However, much less species treated in there.

Theo Zeegers