OK, call to 17070 yields:
This circuit is defined as 01**** ******. Welcome to your OpenReach line test facilities...
So the line is not LLU, and VF is using a BTw line.
@NewtronStar Thanks for that info. I'll probably try it again from time to time, but won't lose any sleep if it doesn't work. Knowing my IPprofile won't help anyway, as I'm already syncing at 40/10.
Interestingly, the download speed reported by 'Ookla' at 20:48 this evening had dropped by 10Mbps to 25.8Mbps, but at 22:02 had regained 6Mbps. Sync has been constant at 39970bps throughout that time, so it seems there has been something of a congestion/contention issue at VF.
It doesn't really bother me, though. Even at its slowest it was almost 5 times faster than the best I could get from ADSL!