Hey congrats!
Your temp NI number can last indefinitely I think. I was on my temp number for almost a year! The tax rate is so high though that they figure you'll sort yourself out pretty quickly.
I wouldn't worry too much about the other job. I doubt it will come up in your interview. The interview will be very thorough though. Mine lasted about an hour, but if you just go and answer their questions, you should be fine.
