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Edit - I missed a point. Thinking again, the fact your data rates haven't change but the noise margin has dropped to 5.5dB probably means you will lose some speed next time you reconnect. Sorry I missed that first time through.
Another correction - it's time I went to bed. The 40000 downstream will stay. The upstream may fall a bit.
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Your data rate, which is the actual sync/connection speed, is still very much as it was. The drop in Attainable is common as other users get connected on FTTC at your cabinet. There is a kind of noise, called crosstalk, often occurs. Crosstalk is where two lines close to each other in a multi-line cable slightly interfere with each other.
If you want to see more about your line, and with luck improve your Attainable figure as insurance against more crosstalk over the coming months, you could buy an HG612 version 3B (which has a white power plug as well as a huge 3B on the label underneath) Openreach modem from eBay.
If there is a locked one a lot cheaper than an unlocked one, get it. If it's only a couple of pounds different get the unlocked one. Depending also on which seller looks best! They take about half an hour to unlock from finding the correct new firmware online, (we can point you to it), to having it up and running. You connect it to the VDSL socket at the master, and run the Ethernet cable that should come with it between its LAN1 port and the WAN port on the HH5A.
If it's a locked one, before doing anything about unlocking it first just connect it as I just described to make sure it works. You won't be able to see any stats from it at that stage, but you will be able to run speedtests to see if its in the right ballpark.
Edited by RobertoS (Mon 22-Jun-15 00:25:07)