@ Murphy23:
Was your Stage 3 on March 22nd? Same thing has happened to me and I'm just as confused and frustrated as you are.
Think it's very poor of NATS not to offer any explanation as to why the interview is cancelled.
Furthermore, I don't even live in England and now have plane tickets, hotel booked and bus ticket, which are all paid for but now useless
Let's hope someone can shed some light on this soon...