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Is that unfairness still running? It seems to have been going on for years.
Another thing to watch for is the order in which such products are used. I don't know how it is for Pay Monthly, but for PAYG it seems to be the case that the one-off Extras are always used ahead of the regular monthly allowance. As far as I can make out, if I buy a Data Extra while I still have any of my monthly bundle allowance running, it immediately switches to using the allowance from the Extra, thus wasting the remaining bundle allowance. When the new bundle kicks in, it appears to be the Extra which continues to bear the strain, which seems only right and proper. So I think it's a case of "Extra ahead of Bundle" rather than "First in, first out"