Notice to Owner will go to the hire/lease company assuming their name is on the V5c
You need to contact them, pre warn them and ask that they challenge using the Hire Company ground and give your name and address (ensure they have the correct one)
Do it in writing.
This should prevent them simply paying or doing something silly like just forwarding the NTO to you.
If they use that gound, it should see their NTO cancelled and a new one issued to you.