Maybe a neighbour rebooted or switched off their modem for 3 minutes, allowing you to experience a glorious 3 minutes worth of relatively crosstalk-free connection.
Would this be conclusive proof that I am suffering quite heavy crosstalk ?
I think it would take a number of similar results that could be intentionally initiated & repeated at will by switching off/on a neighbour's connection before any 'real' conclusions could be drawn.
It might just have been an unexplainable one-off event.
My neighbours are only on ADSL2 at the most, would it improve if they were on VDSL2 as well or get worse ?
ADSL crosstalk only affects the lower of the available VDSL2 frequencies, so your higher VDSL2 frequencies are currently probably not being 'interfered' with.
As an increasing number of neighbours sign up for VDSL2 connections, I can only imagine that crosstalk will increase rather than decrease.
I am trying to anticipate the future as some of my neighbours are looking to get 'Superfast Broadband' so more is yet to come.
I don't think anything can be done to avoid increasing crosstalk, at least until vectoring is introduced.
Just keep an eye open for smallish, but sudden & sustained reductions in SNRM, probably followed a few days later by a resync at lower speeds with SNRM back to 'normal' levels.
That's exactly what I have seen on my connection over 12 months or so, causing me to gradually lose around 1/3 of my DS sync speed.