For lunch I tend to take in salad, with home made syn-free Slimming World Quiche. (You can find the recipe for these on the SW website)
Also "mugshots" are syn-free. If you havent discovered these yet, they are similar to a "pot noodle", except they are served in a cup (like a "cup'a soup"). Very handy if you have a kettle available at work, etc!
Today for lunch I'm going to nip out to the sandwhich shop over the road and get a Jacket Potato with sweet chilli chicken, served with salad. The potato, chicken and salad is all free - although I have allowed 3 syns for the sweet chilli sauce (1.5 syns per level tbsp).
If you have a microwave available at work, why not try taking in some homemade vegetable broth? Chunky carrots, potato, leek, turnip, parnsip, peas..and whatever else you can think of to chuck in there! (Particularly nice with bacon bits, and 100% syn free!)
Hope this is of some help. Good luck x