Advice on MEL required please.

[self-l-freight]

I was a passenger (almost) on a flight which was cancelled due to "a failure in the APU fire detection system".

Could the plane (Boeing, bhx>belfast) have operated without the APU FDS operating or was it essential?

Any additional info welcome as I'm taking them to court for compensation as they say it was "an extraordinary circumstance" necessitating cancellation.