tried to get !MB upgrade, but BT said I had line loss (but they won't tell me the figures, and Plusnet say BT won't tell them!). Checked an old ticket of mine, and Plusnet did a whoosh test which showed:
Downstream ADSL Link Info
Does this figure vary, is it borderline (I think 41dB is the limit for 1MB), can anything be done to improve it?
Nothing can really be done.
You have quoted a the attenuation figure, which is too high for 1MB, if the SNR figure was low as well, you have no chance. (Low SNR figure = high noise)
BT are only obliged to supply a working voice phone line.
You could get another line fitted stating that you want it for DSL, but as already stated, BT are only obliged to supply a working voice line. If the new line still does not meet spec, you are liable to be stuck with two phone lines are still no 1Mb DSL.