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I suspect that even though the amount you had to pay is miniscule, someone in ntl credit control might have to confirm payment is received before any block on your account is lifted and it gets reinstated. It may be a few days before it goes through automatically. Of course that is not good enough. ntl like to go round in circles with this sort of thing, even if it is an error triggerred in part by their inaction.
It's worth giving them feedback via the Responsetek system. It has direct links into ntl at a high level and does receive attention.