Moving from TalkTalk LLU to Plusnet ADSL is actually not that difficult. You'd need to follow the instructions here: http://help.talktalk.co.uk/app/answers/detail/a_id/1...
They basically tell you to ring TalkTalk for free on 0870 444 1820, choosing Option 4, then Option 2 and asking them to "cancel my TalkTalk plan".
I would advise asking them what date the cancellation and disconnection of your services will complete on. Then, call Plusnet straightaway on 0800 432 0080 (also a free call) and ask them to activate your services on the date that TalkTalk told you they'd end, this will ensure minimal disruption to your Broadband/Phone. Also mention that you are moving from TalkTalk Full-LLU and that's the date they told you.
Good Luck and tell me whether this has been of any use to you.
25ms Ping. No Jitter. No Packet Loss.
Perfect for Xbox LIVE.