^^^ Yes indeed, my Ethernet Shield was working fine and then it wasn't. The SD card was in the shield, but not initialized.
I was using DHCP and the project I was working on had me disconnecting/connecting the Uno frequently. I reconnected it for something like the 5th time and it wouldn't acquire an address with DHCP. After messing around with it for a few minutes I removed the SD card and tried again. It acquired an address and connected right up!
Thanks for the advice Tim. I've saved your code for future reference.
Regards,
Jake