Many thanks for the response.
Contact form completed and sent at 12:47 on 16-10.
The last daily usage quoted, for 16-10, is back to normal levels although the running total is still abnormally high.
Just another update for you for when you get to look at this.
I was not here to get Friday's usgage figure but the last available figure for Sat 18-10 looks a little on the high side.
A mod rang me yesterday to say that they have an alternative data storage system that confirms the Usage Monitor is correct.
He further claimed that recorded usage on days when there was nobody in the house could have been caused by back ups to the router.
I've had a better story so far, but not resolved yet: a mod (Patrick) has kindly called to discuss it. Robie has also been in contact to progress the issue as Patrick was away (thanks to both).
Patrick pointed me at the UHD netflix data usage rate and suggested that 7GB an hour may be what my TV used to stream F1 from bbc iplayer (explaining the 23GB usage as 7GB by 3 hrs). Since then I've confirmed that my TV is set to SD (not even HD, never mind UHD) so it should use more like 1GB/hr and I watched for 2.5Hrs, so 2.5GB seems reasonable - 23GB does not.
I've replied with that point and theres some internal dialogue going on at the moment. There was an attempt to arrange a chat to someone for next Tuesday, but I'll be uncontactable for the week. Apparently they will come back to me with something else...
I'll update here if anything significant happens.