"Automatic" document production

Can anyone offer me advice on how to acheive the following:

We want to have document "packs" produced automatically (on the server side)
and saved as single PDF files (this last step is no issue).
These packs will be made up of entire documents, parts of documents and
fields replaced with data.
Users need to be able to create the template documents, and then have some
way of defining how the packs are built/the data replacement occurs.
Each document needs more than one data source, which rules out a straight
Word mail merge.

At the moment we are using word template documents with text (or text areas)
marked as Bookmarks. We then have a "document engine" in C# to replace the
bookmarks with the required data. However, this of course means maintaining
a list of rules in code.
Our latest solution will be to have a "translation" layer, as a web front
end, to allow the users to not only define the document, but also to define
the specific translations, given a pre-defined set of available rules.
Users can select a bookmark in the template, define a rule for the bookmark
and then define the translation/insertion/inclusion etc required for the
bookmark.

Is there a better way to acheive this end result?
I am convinced there MUST be...but I can't think of it!

Thanks,
Dave.
dave underscore chapman at bankofscotland (all one word) co uk