Thanks for the useful comments.
Doing some trawling for Martin Müllen in the last number of Mosel Fine Wines, I see good reviews for both trocken and traditional prädikat wines from 2005 and 1995. It sounds as if they are wines for the long haul.
Meanwhile the website's offers for a 2011 Spätlese trocken from Kröv as well as a mixed case have sold out, so the issue is for the moment academic. It looks as if the users of that website knew something that I didn't

.
The weingut's own website is offering some vintages back to 2007.