It may well be that your old charger is 'comfortable' charging the old phone? it still charges after years of use.
I would do one thing first
Buy a new approved charger and test that in your new phone. If it doesn't work then maybe the charging port is faulty...
*The Game Is On*
If all else fails make an appointment at your nearest Apple store and let them check it out for you. Much better than a diagnosis by Anovo going off the reports about O2's repairers
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