For anyone that is interested in the repair of this problem, the aluminum replacement thermostat housing cover was in stock at my local dealership so I went out an bought it this morning with a new hose and clamps. The part number for the housing cover, which is a "universal replacement" for the plastic unit on W201 and W124 is:

103-203-07-74, and it requires a bolt and washer to seal the air vent connection on a 300E (on my 1988 anyway as the vent connection to the overflow tank goes to the radiator directly), which have the following part numbers:

007603-008106 for the seal ring (copper washer)
007604-008200 for the threaded plug/bolt

If you have a W201 and the coolant system vent connection enters the top of the plastic thermostat housing you need a special adaptor, hollow bolt and two seal rings. I have these on order for my 190E 2.3-16 as, after this fiasco I checked and that car also has a plastic cover.

By the way, the cost of the new housing cover was $10.00 plus tax (CT sales tax is 6% on this item). I wish I had known about it when I bought the thermostat. I hope this helps someone else - remember to check the thermostat housing wcover hen you buy a thermostat, and replace the hose and housing cover at the same time! Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)