The problem – I needed to combine 6 split 7z files: file.7z.001, file.7z.002, , file.7z.003, file.7z.004, file.7z.005, and file.7z.006. I was unable to do this using p7zip although perhaps someone could explain how. Anyway, I have two solutions:

Solution 1: The command line
As per: http://ubuntuforums.org/showthread.php?t=859596.

e.g.

cat file.7z.001 file.7z.002 file.7z.003 file.7z.004 file.7z.005 file.7z.006>> file.7z

Open the resulting file and extract its contents. NB if any of the parts are missing you will not get a file you can open.

Solution 2: WINE and the Windows version of 7-zip

Install the windows version of 7zip and run it under WINE. Then shift the 6 files into the ~/.wine/drive_c folder, and open 7zip. Navigate to c drive (actually ~/.wine/drive_c folder ;-)) and double click on the first file (file.7z.001). Then extract its contents.