FTTC 15 Mbps down, 0.4 Mbps up

[#1]

Since my FTTC connection in August there have been problems with the speed.  To cut a long story short the estimate was 31 down and 5.8 up, the actual at connection was 9 down and 0.49 up.  Since then and 10 BTO engineer visits the download speed dipped further to 7 and has now climbed to 15, however the upload speed has stayed fairly static at around 0.45.
Can anyone shed any light on why the up speed should remain so very low despite the current improvement in the down speed?
Yet another BTO visit is planned for next week but I would be interested to know if this disparity is a known issue. 

[#2]

Hmm, it looks as though it is not a known problem.  I thought the disparity was somewhat odd but I assumed that it must have cropped up in the past.