Thanks for the suggestions. I routinely sweep for viruses and spyware, and there are no other applications running which are using bandwidth. Windows task manager and my ADSL router both say there is no network usage at the time I start the speed tests, so I'm confident its a fair test. I'll contact Plusnet again and see if they can sort it out.

Yeah we stopped the migrations on to LLU a fair few months back now. Have we raised this as a fault with BT? If not let me have the ticket number and I'll take a look into it.