Thank you for your thoughtful help, Kitz.
Catch-22 comes to mind.
By the way, when the tech support person realised he had no idea what I was talking about, he went off to speak to a colleague who told him that vectoring would have an effect 'only on long lines' and would therefore make no difference to mine. That sounds to me as though they were confusing vectoring with long-reach VDSL. But I'm not knowledgeable enough to win an argument with them on the technical aspects.