Excellent, Ron! Thanks so much for your help. This has done exactly what I asked.

Just one thing I am wondering though, once I've used up all of the letters of the alphabet (let's assume that I have one marble lot that is, say, 50kg) it starts going through symbols and lower case letters. Is there a way to get the spreadsheet to go ....X|Y|Z|AA|AB|AC.... rather than going through symbols and lower case letters?

Thanks again. You've just saved me days of work across a whole year!