It looks like we might have an issue in the house after all, switching everything off in the room except the router just about stopped the CRCs completely, although strangely the FECs continued unchanged. Re-synching at 20:00 gave a slower speed and at that speed with everything switched back on I'm getting a pretty low error rate, something like 0-6 ES per hour (total 82 in 23 hours).
What seems to have changed is the SNR now varies more between day and night, when we started getting really bad errors last night it was varying between 2.5 and 3.5, down from 6dB at 16:00. So naturally a re-synch at that time dropped the speed pretty severely.
So fixing the audible line noise has made a big difference, but not completely solved the issue.
