Nick,
On top of the excellent diagnosis steps and suggestions other have provided, as another Plusnet customer (sufferer) I can say that yesterday my connection speed was very poor despite my ADSL sync achieving the normal circa 7Mb/s.
Plusnet have several faults but speed isn't usually one of them, but yesterday I had lag all day on my irc connections, webpages loading felt like they were on dial up (or at least ISDN) and my SIP phone spent most of the day losing and re obtaining connection to my asterisk server at the office.
It was like this all day from about 11AM until I went to bed at 1AM Monday Didn't look at is this morning but all seems fine enough tonight. So perhaps Plus were having some problems they have decided not to admit.
As to others comments on Plusnet customer service, they aren't too bad at the moment it seems but have been terrible and woefully incompetent in the past.
You have to worry when the Customer service levels actually *improved* after BT bought them. :-)