Well, unsurprisingly, the problem has not gone away. ??? Occasional clients are still getting the rogue gateway.
There has never (intentionally) been a DHCP server running on the 3G client. This client is a Windows XP Pro machine, and looking in the control panel the only DHCP service I could see was a DHCP Client service which appears to run on XP by default.
Also, as I mentioned earlier I could see the DHCP handshake - request, inform, ack, etc in the pfSense DHCP logs, showing the IP the client received, but then an ipconfig /all on the windows client (not the 3G one - just another client on the network) showed the DHCP negotiated IP from pfSense, but the gateway IP is the one of the 3G client.