PlusNet have decided to remove it's fair usage policy, which seemed overly complicated compared to other ISP's. But it is confusing matters by introducing a revised by a contention based system, with Premier customers being moved from the standard 50:1 to 30:1 radio.

The reason for this is not straight forward, Plusnet claim it's because of  "the changing broadband market", but most users are trying to figure out what and how the new system will be implemented.

In the meantime they have posted a rather long, but lacking in detail, help page with some common FAQ's.
Hopefully further news will clarify the situation.