Given what Kirk has posted further up I wold say that DLM is gradually bringing your line down to what it believes is a stable connection. What estimate was given on your line as the initial speed will be higher as the line is put into "go as fast as you can mode) albeit with potentially errors occuring which may cuase DLM to intervene.
As to your current throughput that may well be lower as its possible that at point when you werent checking it the IP profile dropped alot and but has recovered. If the IP profile was matched at plusnet's end then your current PPP connection may well still be running at this lower speed. So it might be worth while dropping the PPP connection the 582n router has established over the OR modem, waiting 30seconds and then connecting again. As others have said many a time on countless threads don't just turn off the router and certainly not the OR modem as this can make DLM think there is issues with your line,
Also, it might also be interesting to see Radius chart to how many disconnections you may have had since fibre installed
