Fibre installed, speed dropping every day.

[#1]

I am very grateful that fibre is finally available. Placed the order and the engineer arrived and installed without any problems. 
After the engineer installed everything, we did a speed test and my results were 32.6 down and 6.44 up, about 3 weeks later my speed is now 15.84 down, 4.26 up.
I find it strange the speed has dropped so much, my line is only 3 years old (new build) and it is copper (I asked the engineer) the cab is about 0.8 miles away using google maps following the road… The master socket was replaced with a new faceplate.  Any ideas? Could this be a fault? Thanks
(BT Wholesale info on my number below)
                                                High Low High Low
FTTC Range A (Clean)         28.6 20         6.6         4.8 -- Available
FTTC Range B (Impacted) 22.4 10.7 6         2.6 -- Available

[#2]

This is normal behaviour as a cabinet fills up. Other users will cause crosstalk in the cabinet, and can knock your speeds down a bit.
As long as your speeds are in line with the estimate shown on the BT website (which they do change as your cabinet fills up) there's nothing you can really do.
As your speed is lower than your estimate, it could be a fault.