Recently had plusnet installed, currently terrible speeds.

[#1]

Good luck - I hope it gets sorted for you.
Just before you disconnect the router again, grab a copy of the stats and hang on to them just in case they are needed.

[#2]

Quote from: owls10

No change in speed after connecting to the test socket,

That's to be expected. What they will be looking at is the underlying errors that are causing the speed to be restricted. Only after the errors stop happening can the speed be restored to normal.