Andy... I can understand that. It happened to my Golf once. I don't know for sure if it was the sunroof drain... but I parked it on an incline (front left was downhill) and there was more than an inch of water on the floor.
However, I must say that there are good sunroof designs. I have owned a few Mercedes... and worked on hundreds of them. I have never seen one with a leaky sunroof. Yep, even ones with over 400k miles on them (like my last one). Mercedes outsourced their sunroof work to Webasto. The first thing I did when I got in my new Golf (back in 2002) was to open the sunroof. I looked around for a "Webasto" name on any of the parts and was somewhat dismayed when I couldn't find it. Oh, well...
PS: I never had to clean the sunroof drains on my Mercedes. I owned it for 6 years and 210,000 miles (it already had 200k on it when I bought it). It sat outside under trees the whole time I owned it... just like my Golf. A good sunroof design makes all the difference.