We are now connected with FYX and everything is working fine. Our connection speed has dropped from 8Mbit with Vodafone to 5.5Mbit with FYX (due mainly to the setup of the Telecom / Chorus hardware in the exchange).
The support desk has generally been pretty good although they are only available to talk with during business hours.
The change to turn interleaving off actually seems to have made our connection rate slower. Now we are connecting at 5.0Mbit with line attenuation of 29dB and noise margin of 12.3dB. Throughput is slightly down from tests done to the geekzone speed test server last night.