Ok, thanks for all of the replies and suggestions.
I did have a problem recently with a VERY noisy phone line, which resulted in 3 separate Openreach engineer visits, where they gradually replaced all my wiring from the buildings Distribution Point to my Master Socket and replaced it with a new model 5C. I live in a building containing 12 flats. The second engineer thought that he had found something at the street cabinet and rectified that, but he wasn't totally happy with the outcome that had been achieved. He then returned the following day and did something (finding a spare pair) in the underground manhole adjacent to my building. After all of these visits the noisy phone line seems to have disappeared, and I believe that a DLM reset was carried out at the end of the above work. This all happened around July / August last year.
Now, yesterday, I tried putting an Ebay VMG1312-B10A on to see if that would connect at a different rate, but I couldn't get it set up properly, so I have now reverted to my 8924-B10A, which has connected with the following stats.
DSLAM type / SW version IFTN:0xb206(178.6) / v0xb206
VDSL Profile 17a
Down Up
24.5 Line attenuation 0.0
32400 Connection speed 5279
6.3 SNR margin 6.1
0.9 Power 0.8
687 Interleave depth 1
3.5 INP: 0
I took note of the stats prior to introducing the VMG1312-B10A yesterday, and they were almost identical to the above apart from
24.6 Line attenuation 0.0
32400 Connection speed 5648
4.8 SNR margin 6.1
1.0 Power 0.9
1039 Interleave depth 1
3.5 INP: 0
I'll try the ISP DLM reset approach, as licquorice says the underlying fault (whatever that is) needs to be resolved. Any suggestions on what the fault might be.