Hi,
I’ve come across something that seems to do the job – look for the MM Forms plugin in the WP extend directory. It’s built on Contact Form 7 and seems pretty good. It writes to a db file and sends an email when the form is completed.
The developer seems to be struggling to answer questions, but here are the things I learnt using it:
a) Most of the advice for Contact Form 7 applies to this plugin as well (in terms of making the form work etc)
b) You have to manually create an exports directory underneath the MM Forms plug in directory
c) You need to tick the export ID option to get the CSV file export to format correctly.