Its sounds odd tcoys, but if you don't eat very much, your body holds on to everything you do eat and stores it as fat. Its holding on to it because it knows there won't be anymore food anytime soon.
My suggestion is you take some food with you to work. No more crisps, cereal bars, snacks or paninis but have some cereal and a piece of fruit before you leave in the morning, and stop for lunch if only for a short time.
For lunch have some soup or a salad you've taken with you, or ryvita and lean meat or low fat cream cheese, with a tomato and some fruit. How about sushi, that's filling but low in fat.
If you fill up a bit more with healthy fruit and veg and cut out any snacks, bread, cheese, cakes etc your body might feel inclined to let go of some of the fat its holding on to for a rainy day!
I can only say that if I can do it, so can you. I still have a long way to go but buying new (smaller) jeans recently was the most exciting thing I'd done in ages!!