The problem this time looks to be that you're using a... err... some weird version of berkdb that doesn't have Db->exists() !
I recommend installing berkdb 4.7.25 -- that's the version the pre-compiled bitcoin client (mine anyway) seems to use

Thanks, I'm starting to get very frustrated with this now, I still am not even close to get it working:
I have now removed my previous verison of berkdb (with "make uninstall") and installed berkdb 4.7.25 as you suggsted. This resulted in the following error:
net.cpp:22:32: error: miniupnpc/miniwget.h: No such file or directory
net.cpp:23:33: error: miniupnpc/miniupnpc.h: No such file or directory
net.cpp:24:36: error: miniupnpc/upnpcommands.h: No such file or directory
net.cpp:25:34: error: miniupnpc/upnperrors.h: No such file or directory
In file included from main.h:11,
from headers.h:96,
from script.cpp:4:
db.h: In member function 'bool CDB::Exists(const K&)':
db.h:149: error: 'class Db' has no member named 'exists'
In file included from main.h:11,
from headers.h:96,
from net.cpp:5: