From looking at your IP address in your posts, you have the same IP now as when you made your very first post to the forum
here, so I think its a sure fire bet that you have a static IP.
When I joined Plusnet the first time round (2003) you were automatically issued a static IP. If memory serves correct then it was only when they started with the Basic and BroadbandPlus accounts circa 2005 that they changed things and the cheaper accounts got dynamic IP. iirc the Premier accounts still got a static IP by default for a while after that.
Re the change of your IP now. I saw something on their forums pretty recently saying that they were trying to free up some IP blocks and to do this they are having to re-assign some of the really old IPs.