As I breifly mentioned in my earlier reply to Mike, it was because I never bothered checking the speeds for the various serial standards.
Initially I had considered using Modbus in RTU (serial) form as interfacing to a PLC could be useful, but it's usually limited to 56kbps, which is no where near fast enough. I did briefly look at Modbus TCP (ethernet), which has more than enough speed, however it seemed overkill for what I'm trying to achieve, so my next thought was CAN bus. It's designed to be tolerant of external noise, but now having checked the speeds for the other differential serial options, RS485 with a custom protocol looks ideal.
I don't need multiple nodes on the same bus fighting to send a message, as there will always be a master, so even if I do go for multiple slaves, RS485 should have enough speed.
Anyway, thanks for the replies everyone.