I cracked it - the development environment doesn't have python2.5 installed properly, so I had to copy over everything from the standard /usr/lib/python2.5 into it, and for good measure I also installed python2.5-qt4. I then could build a proper .deb which works perfectly, no need for faffing with the source tarball any more

Thanks for the help pycage, and the great app.

PS I didn't check out on my device, but used TortoiseSVN and then copied the files over. For this reason I had to modify the make-deb.sh to include "_svn" as well as ".svn" because it's a Windows machine on which I develop with Visual Studio just didn't think it was worth mentioning.