Ok, im not 100% i am executing the merge as your making me think im not. ill explain in a bit more detail.
1. our .dot template has a retained link to our .mdb so no need to select datasource
2. open .dot and open query options then specify "client ref" equal to "d150"
3. in word on the mail merge tool bar we select "merge to new document"
4. this then produces a new document with our data we requested.
5. save the document and then send a copy to our client but when they open it ask's for merge.mdb.

Hope this helps as i am out of ideas