Most of the way there. When I type the command I get a long list of the contents of my home directory rather than just one entry.
Taking lots of care as root in dolphin find the problematical tmp directory and right click, look at the permissions. Should be in the name of derek and not root. Change to derek if this is the issue.
Also are there a lot of directories in /home/derek, there should be, and they should be in the name of derek. They should have been listed in the command Andy suggested. Slightly puzzling.
ps Another thought. Is your home directory new? Doesitt contain any files you need or have saved? In which case go to the Control Centre, users, oblitorate user derek AND the home directory, make a new user derek to include a new home directory. CARE - if you obliterate the old user all files in that users home directory will be obliterated. Dirty fix.