Sessions not working on Linux - Apache

Running on apache 1.3 session.save_handler = files session.save_path = /tmp session.use_cookies = 1 session.name = i session.auto_start = 0 session.cookie_lifetime = 0 session_start() is included in the top of each page. When I test session files are being written into the /tmp folder and the session variable is corerctly added to the url when moving to another page, BUT the sessions do not appear to be working? I can set a session var on one page and display it there but when moving to another page the session var is no longer available? If I look in the tmp folder a relevant sess_nnn file is there but contains no data. I've checked permissions and the /tmp folder is world writable and the sess_nnn file is owned by the web server and is writable? Can anyone suggest what to test next or where the problem may be? Thanks Zac