Hello Frans,
I have looked on the Hannover soldier records which are kept in England and unfortunately there is no mention of the name SOMMERLADT. The only names which are recorded are 1 x Sommerlad and 1 x Sommerlat (both in the wrong time-span). These two spellings are also the only names that appear on rat de. and at least one person is researching the same time-span as you.
http://www.rat.de/kuijsten/navigator/germany/index.html