Somehow I missed this extremely helpful post.
My current quota, according to the machine-readable friendly service at
https://quota.aa.net/, is 4.081 TB. That is total, not per-line, I assume, but I don’t know. I have three ADSL2 lines and somehow AA has just done the sane thing and set the system up to share the quota between the lines. This new tariff is brilliant as the old ‘units’ one I have been using for eleven years was very very expensive in the daytime 0900-1800 Mon-Fri and now the usage is time-independent.