They look they might be files that are created by PHP scripts that are running (I'm actually not sure, but, least the sess_* ones). It's unusual for something to create that many files in /tmp. You could try moving them out of /tmp, rebooting, and seeing if it broke anything. If it broke something, move them back, otherwise, remove them.
Thanks I cleared the tmp directory using ssh and the space used went down to 16k, just the session files however I checked again a few hours later and the size of the directory had increased to 3.5G with the same time of files, any ideas on how to prevent these files from being made?
My host offers "/tmp & /var/tmp hardening – We harden /tmp and /var/tmp so no malicious scripts can be executed from the commonly used directory." but you have to pay for it, can I do it myself and will it help solve the problem?