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You are going to need to either phone customer service on 191 or use Live Chat and they'll access your account to see what they can do.
Current Phone > Samsung Note 9 _512Gb > Model: SM-N960 Hybrid-Duos.
Samsung One Ui / Pie.
Samsung Gear s3 Frontier Watch.
Although it won't do any harm asking, if the credit has been added, it's unlikely Vodafone will be able to change the top up. When you topped up, the option would have come up on the screen and you would been asked to confirm your choice of a £5.00 top up by clicking again on the transaction.
You will more than likely need to add a further top up.