You are completely correct! I actually came back to comment I'd just found the solution - I hadn't chosen the x64 version. Works perfectly now!
I've just installed CODESYS and I'm following the Installation and Start PDF. Everything is fine until step (8) in 6.3. Scan Network isn't detecting my computer. Gateway is running (with gateway started), Control Sys is running (with PLC started), CodeMeter Control Center is running (if that matters). Wireshark is showing loopback traffic to 1217. I noticed the Wireshark traffic is TCP but Gateway Configuration only has Default UDP interface, but adding a TCP interface makes no difference. Firewall...