Note: IL6 engined E320 CDI - i.e.straight 6, not V6. Mine is 2004. Thanks to Colin (SAM/Unit) for the advice. The grid/drain located in the engine bay between the wipers was blocked. Here's what I did. Removed the pollen filter exposing the pipe that goes from the drain between the wipers to a port in the nearside inner wing. The pollen filter body had been fowling the pipe and had cause a kink which was preventing the water from flowing (see 1st photo and chalk mark showing kink). Additionally, the pipe had come adrift from the port in the wing (see chalk mark on 2nd photo). I suspect that the refit of the pollen filter body (following a filter change at service) had caused both problems - kinking the pipe to the left and pulling out the pipe from the port on the right. Because the rubber pipe was now deformed by the kink I slid a jubilee clip along it and tightened it enough to reform the pipe. Then I re-inserted the pipe into the port. I refitted the pollen filter ensuring it wasn't fowling the pipe. Tested and all seems ok. I haven't mastered Photobucket yet so if you need a better picture PM me and I'll email it to you.