When, precisely, did Avanti use the word “fraud”?
They suggested they had claimed funds they were not entitled to; that's good enough for me, and good enough for the OP, who knew full well what they were being accused of.
If the police call me in to the station for an interview about my finances and say at the outset "well, look, we aren't charging you yet, but based on your interview, we could proceed with a charge based on the fact you have claimed refunds you shouldn't have", that's an accusation. The police could voluntarily interview me about fraud and never once use the word fraud yet the context be entirely clear what they are asking me about.
I'm not going to get into a pettifogging argument over whether Avanti used the F-word. The recipient of the correspondence knows exactly what is being alleged and why it's been sent. As would the man on the Clapham omnibus.