Sorry, Amn't I silly not to have put my name down. I'm that last guest.
People use "amn't I?" all the time in the UK unless they are very posh.I think they do it without noticing it .Were they to analyze it,most people would probably say that they were using bad grammar - misusing the words "aren't I?"
"Mum ,do you love me?"
"Of course I love you ,I'm your mum amn't I?"
Nobody in the UK would say , "Of course I love you ,I'm your mum ,am I not?" .