This probelm is very specific to the Fritz!Box7390 (and possibly only for users connected to a Huawei cabinet), I just changed to a 7490 and the problem of ~90% loss of upload speed has gone.
The problem happened (for pretty much everyone reporting it) near the end of October, I was still running Fritz!OS 06.20 at the time and later updated to 06.30 which made no difference - I can only assume that BT changed something (remote update) on Huawei cabs at this time which just happens to have impacted all 7390s connected to them by VDSL.
I earlier reported the issue to AVM and provided a DSL line test log on their request, I've now submitted a log from the 7490 for a direct comparison on the same line.
I got a reply from AVM today thanking me for the additional info (log from 7490 for comparison with 7390) and they say the investigation is ongoing.
Received access to and installed beta firmware as of yesterday 13/1/2016, works fine, back to 62d/19u. So I guess the support turn around is 1 week, that includes the time taken for me to collect the requisite data.