Any reason the IT dept are no help?
1. Have you tried Ethernet anywhere else besides work, if so does it work there?
2. Check the Ethernet socket on the Macbook for broken bent pins.
3. What speeds does the Ethernet at the office work at 10, 100 or 1000mbps? The 1000mbps is much more demanding of the hardware/cable, if there are problems it usually drops to 100mbs but not always and the connection fails.
4. Is the Ethernet port showing disconnected in System Preferences/Network?
5. If it showing connected is it not getting the correct IP address for you office?
6. Does your office run any security software on the network that stops unknowm MAC addresses.
I have seen cases where for what ever reason the ports on the host and client will not match and connect, I know they should but if all else fails try putting a small 4 port switch between your Macbook and the office Ethernet socket. Only point in doing this if the Ethernet port works elsewhere.