Whenever I make a flight booking or change I always print out the confirmation. This details the amount paid, ticket number, avios (if used). I've found this very handy as a backup recently in case of BA IT woes. Had an avios reward flight saver and got the money back, not the avios. Had the printout to hand calling them on the phone (no wait at all BTW on gold line) and the missing avios were instantly returned.