I just had my Fibre Unlimited switched on yesterday, coming from TalkTalk LLU. Most problem-free I've ever had it, actually, and zero downtime except for a few hours on the day the phone was switched back to the BT network simultaneously with the fibre activation.
I simply ordered the service from Plusnet, and didn't have to communicate with TalkTalk at all (minus the several calls from them to both my home phone and mobile that I ignored once I knew the number was theirs, because they obviously wanted me to agree to stay!).
All done within just under 3 weeks, and enjoying the speed!