There has been no contact from Virgin on this, but following a loss of service all day yesterday (good signal - but no IP address could be obtained by either my own router, or the VM router if I set it back to router mode) the old IP I've had since moving in changed, and there is very little packet loss now.
The CMTS MAC hasn't changed since September, so maybe the IP change brings this to the end state of a re-segmented network.
Thanks for giving us an update on this issue with your internet connection, I am glad to hear things seemed to be smoothing out the last time you posted thanks to an IP change. Can I ask if things are still looking okay now its been a few days?
If its began to go downhill again, please show the results of a recent BMQ test and speed test so we can see what your connection is like.