The FEC errors do look high for the connection time, but you really need to rule out wireless issues first. The PN supplied router has a pretty poor reputation for wireless.
If it is a wireless issue, you may not actually have a problem with your connection from the router to Plusnet - yet. Keep rebooting the router and it is highly likely you will have as the exchange will think the line is unstable and slow your speeds at all times to try to make it stable.
Plusnet user since November 2001 - not sure for how much longer