The new supplier sends the opening/closing read to the old supplier. Basically, when the customer passes to a new supplier, the new supplier asks for an opening read (as we do). This read is then sent off to central industry. If it gets validated then both the old and new suppliers get an agreed read back.
However, if validation fails then the central industry will produce an estimation instead and will send off to both suppliers. Finally, if a customer does not submit an opening read at all, central industry will produce an estimate and send it to both suppliers.
Sometimes this can take a little while to come through (either to new or old) and so can be hard to bill. You may notice an estimated bill in your last bill from your previous supplier and this will be why.