He said the target noise margin is between 6-15db and at 31db this is double which indicates too much noise so my line has been fixed to prevent it going even slower
That is just so much total bo***cks!
Firstly, it's a logarithmic scale so 31dB is NOT twice 15dB.
The actual ratio with those numbers is more like 40 to 1!
And it's the other way round, so a 31dB noise margin means a VERY VERY VERY low level of noise. Which is GOOD.
Compared to my 3dB it's actually about one thousandth of my (relative) noise level.
(and 327dB would just mean the noise had effectively disappeared briefly, hardly a problem!)
Get thee to a decent ISP ASAP!
PS With a fixed speed line there's no such thing as a target so more drivel being spoken there!