Hi Clive
For your attenuation, you should be syncing at a much higher speed. However, your target noise margin appears to have been raised from 6db to 15db. This is done by the line management software when it thinks the line is unstable, lots of disconnections, and each 3db step lowers sync speed by 5-700k, so you've lost about 2000k from what your line should be able to support.
This may just be a characteristic of your line - overhead lines in particular are susceptible to noise pick up, and this can trigger the re-syncs. However, if you can maintain a constant sync for 14+ days, then the target noise margin should reduce by 3db, and repeat the process until it reaches 6db again, or the line becomes unstable.
As Sandra says, connecting at the test socket will eliminate any local issues (if you have any extensions), and as Simon says your throughput is a little low for the profile, but not unduly so. If it's any consolation, I live in a town and have worse stats and a slower connection.
Don't worry about testers giving different results at different times, there is a number of factors which affect the results, as all testers 'cheat' to produce a figure, and a small change in speed will usually be amplified as a result. The only one which really matters is the BT one.