This is the link to SEGA Europe Support:

SEGA Europe - Support Knowledgebase

I think you've already seen this. If you set Product to "PC" and Category to "CD Key," you can then choose a subcategory of Medieval Total War. However, I tried it, entering "invalid" as the search phrase, and the knowledge base returned nothing! .

You can create an account from the Login tab. I would do this, because once you do, it may provide you with a way to enter your problem on a screen, or it may provide you with an e-mail address.

SEGA Europe - Support Login

I also found SEGA USA:

SEGA USA - Support

If you can't get results in Europe, try USA. the one thing I read in several posts at the .com is be persistent. "The squeaky wheel gets the grease, etc." Good luck. You know how to reach me if you need to.