As I noticed that BT 21CN was now available on her exchange and that she was already on a WBC line, I requested a regrade to the equivalent ADSL2 product; this regrade happened today.
Since then the router has been syncing at 16Mbps down (I can confirm this via the "Line Rate Data" section in the Zen portal), however there is (and this is where it gets vague as I'm not on site any more) either no, or a very flaky internet connection.
Looking closer at the "Line Rate Data" in the Zen portal, I can see that the router has been continually re-syncing every 5 - 20 minutes between 5:45AM and 11:46AM this morning (I'm not sure if it's still doing that, or whether the line data has a several hour delay on it) which might explain the apparent flakiness of the connection.
The router in question is a Netgear DG834Gv3 which this thread reports to have issues with ADSL2.
Is this flakiness and constant re-syncing part of the normal training process after a regrade, or is the router just having issues?
If it is I'll probably just replace it with a Technicolor TG582n from Zen to save her any more issues.