Thanks for the reply!
I've checked that the formula is text, and it's still not working...
This is the formula customised for my spreadsheet - is there a mistake in it?
=IF(AND(LEN(O31)=1,ISNUMBER(-O31)),TEXT(O31,"\OO0"),IF(AND(LEN(O31)=2,ISNUMBER(-O31)),TEXT(O31,"\O0"),O31)))))