Do I have to pay £330 for new airline ticket to change name?

My passport says I'm Thomas, but I booked the flight as Tom; surely the airline should not make me buy a whole new ticket I booked flights to Chile via Brazil and used the name Tom, whereas my passport bears the name Thomas. This was not a problem on the flight out to Rio, but when I checked in for the Rio to Santiago leg I was told that since the names didn't match I would have to buy a new ticket at an extra cost of £330. I work in fraud detection and would never make an issue over such a d...