Just found a helpdesk program which does what I need it to do.
You can get it for free (with 1 technician licence).
Help Desk Software by Web Help Desk
I have it installed on our intranet and it seems to work perfectly, installed and set up this morning (users can be imported from SIMS), and I have already got 30 tickets waiting, so its a working system.
Thought I might share with you all.