For what it is worth I spoke to TW today who re-iterated their assessment that the cause of the leak was on private property and thus it fell to the owner/freeholder to arrange for its repair.

TW understand a contractor has been commissioned but were unable/unwilling to tell me who they were or when they understood the work might start (citing erroneously the usual rubbish about the information being protected by Data Protection Act).

Seems TW does have authority to intervene if the work isn’t carried out within a certain – undefined – timescale. Their registered first notification about this leak was 1 May.