The trojan is a classic PlusNet excuse
From the comments above PlusNet seem to be pointing at a Trojan on customers machines as a likely source of problems. I'm not a PlusNet customer and haven't used their email system for months (I occasionally check my old account from a couple of years ago).
This week I started getting identical spam to three email addresses that have not received a single piece of spam in over 4 years. One of these hasn't been used for over 2 years. Logging into Plusnet's webmail system there were no emails in the inbox containing the 3 spammed addresses. However, I found that all three addresses were setup as identities in the webmail system (the webmail system allows identities to be setup, each with a different 'from' address).
This looks like the spammers have had direct access to the backend of PlusNet's email systems (this is the only place where those three address are listed together) and I expect that any addresses listed as identities or in addressbooks will have been collected.
Biting the hand that feeds IT
