I didn't mean to imply that the change to cat5e was solely responsible for my improvement.
The first three OR engineers who came out said that the line from the master socket to the cabinet was fine and that the problem must lie within my home network.
Fortunately a fourth rather more clued up engineer diagnosed an HR on the line which had not caused a problem with ADSL but ruined my VDSL. This guy swapped me on to a spare pair which was luckily available in th UG line and th improvement both in speed and stability was immense.
Having said that my new internal wiring may have contributed to the improvement.
The main point I was making is that the switch from ADSL to VDSL can show up pre-existing faults that did not cause a problem with ADSL.
A further point is that not all OR egineers have the same degree of competence.