Keep in mind that his "new supplier" or old, is absolutely dependent on what is available on the newer FTTC Cabinet, which seems to be referred to-
" it appears that there's no capacity in the cabinet for him to get fttc - which is irritating as he's no more than 10 yards from it."
My local FTTC a similar distance from my front door, has an eventual total capacity of 288 FTTC/VDSL connections.
It went live slightly early in March 2014, from on-site work started November 2013, with only 48 of those connections available, ie they had the small Filter/Links installed in the first group of the (6x48=288) back-plane.
The second group of 48 was added in June 2015.
So in its case, depending on when the groups of F/Ls are added etc, there can be a number of occasions when new VDSL circuits are/could be "temporarily" not available.
And there is a lot more to upgrading to so-called Fibre, which is actually only Fibre to the FTTC, relying on the existing "D-Side" copper from the older PCP, Cab 17 for that case, to the user, plus the new double-links from the PCP to the FTTC to carry the VDSL signals.