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This is Scatella tenuicosta, not stagnalis. It is the commonest glasshouse species. In fact stagnalis has never been found in glasshouses, despite the many references to them there. I am preparing a paper on this subject (in my "to do" pile! :o).
The distal wing spot is often shaped like an hour-glass in tenuicosta, but is either square or rounded in stagnalis, which is a bigger, browner species as well.