Phone peugeot, ask the parts dept for a price, then say you'll give the deatils to your wife to order it and can you have the part number........... simple
didnt think of the wife trick, tried phoning them, and they seemed less than keen to disclose the info, dont see why when its printed on the starter anyway.
Don't put too much faith on the number on the starter. It might be the manufacturers number, i.e. Bosch. Also, the car manufacturers update things from time and you get a new part number.
Have a look on ebay under '406 starter' or similar, they often list the original part numbers. Also try looking up other cars that use the same engine.