Lastnight I disconnected the battery for a few minutes, thinking it might reset the computer and clear the memory so that I might get an accurate fault code.
Now the radio won't work. the securitylight keeps blinking on and off. But the radio won't come on. It powers on but I get no sound.
Is there some kind of reset button that will get the radio working again?

When you disconnect the bat , you loose the radio .
You have to re-enter the radio code when you hook power back up.
Look in your paperwork for the code and instructions for entering are in your manual.
Or Archieves ...

Always make sure you have your code handy before dis-connecting power.

I don't know if you're using Mercedes brand but if not, some have a reset button that must be depressed after you re-connect power. My Sony works this way.