Pull the other one Bob
"Plusnet takes its obligations regarding our customer data very seriously"
They really don't, and have a long, long record of not doing so with customer emails going back at least ten years. I set up an email address for my parents, solely for plusnet billing purposes, that's been getting a ton of garbage as described since mid Nov. Since none of the other addresses on the domain are getting spammed I can only put it down (yet again) to Plusnet.
Its long overdue for PN to clean up their act in this respect, and to come clean when they do screw up yet again. So I won't be holding my breath, but I'd love to think the ICO would take them to the cleaners for what is quite obviously down to them.