3holelover, its pretty obvious that the OP is dealing with an Irish airline which is rumored to deny ALL claims in the first instance.
What you might WISH to be the case is irrelevant when the law has something to say, and you seem woefully uninformed of EU law:
Cancellation
Financial compensation is due unless you were informed 14 days before the flight, or you were rerouted close to your original times, or the airline can prove that the cancellation was caused by extraordinary circumstances.
I dont think an inop APU on a European internal flight is extraordinary - and neither does the court!
In a landmark judgment, the European Court of Justice said that airlines must pay up unless the technical fault was due to exceptional circumstances – such as terrorism or sabotage.
The ruling was a victory for both consumer groups and the European Commission which has accused airlines of frequently trying to sidestep their legal obligations.
Passengers win right to compensation when flights cancelled for technical faults. - Telegraph