Well. I started a conversation with Thomson support some weeks ago, and it has progressed regularly. I have to say I'm actually impressed with the helpfulness and professionalism.
The end result is that they tested on a modem with 8,.4 firmware; the problem still exists, and they're not sure why. I gather from some other places that the firmware is based on something from Broadcom (but I could be wrong) so it may not be under Thomson's/Technicolour's control.
So they have confirmed that it is a problem, and currently we can't get the actual volumes.
We just have to live with what we have, until something else turns up... At least I have closure and can move on with my life
Cheers,
Andrew