It is possible that you are being affected by crosstalk. When vectoring is eventually deployed this will alleviate the problem.
Openreach are concentrating on rolling out VDSL services ATM.
As VDSL2 (currently used for FTTC connections) uses the higher frequencies, then these signals are susceptible to interference, known as crosstalk, from other VDSL2 broadband signals being carried along adjacent "copper" line pairs.
More info available from here: http://www.increasebroadbandspeed.co.uk/2014/vectoring-crosstalk-crisis
There are new technologies being developed and tested to combat such interference.
From looking at other FTTC broadband subscribers over the past couple of years, it is not unusual for the sync speed to decrease slightly from the initial training period. Potentially due to cross talk and other interference.
It's always best to check the data fromOpenreach via theBT Wholesale database as this will give you a more realistic estimation of what sync rates may be achievable before signing up. The data in the BT Wholesale database http://dslchecker.bt.com/adsl is from Openreach and is regularly updated.
Edit: just seen licquorice has posted the same link.