Thanks for the response,
We tried the Static IP after the DHCP kept failing, only to find, that while the display on the S5 is showing the static IP, it's no longer visible and not responding to pings.
I'd previously swapped the ports and cables between the S5 and our 3 Extended to rule that out, but haven't tried swapping them back since the static IP was put in place, so I'll try that today.