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I’ve still not been able to get hold of someone
Yeah, last week of January is not the time to be calling HMRC, and they've been even more shit than usual since most of them are WFH. Just pay what they're asking for and sort it out later. If you don't have the cash and it's a relatively small sum, you can arrange payment by instalments over a period without having to get actual human intervention.
I’ve still not been able to get hold of someone to confirm my calculation is right - do I just need to assume I was under taxed and that the number is right?