Quote:
"If you accidentally pay into more than one in a year, don’t attempt to fix it yourself, as you may close the wrong ISA. Instead, call HMRC’s ISA helpline on 0300 200 3300 to get advice on what to do.
There is a similar process if you accidentally paid too much into an ISA (so more than £20,000 for an adult ISA, for example). HMRC will work out which ISA had the payment into it that breached the limit and will reclaim the money (including charging you for any tax owed)"
Source:
https://www.sharesmagazine.co.uk/article/watch-out-avoid-these-10-isa-mistakes-now
So now all you've got to do is to actually get through to someone on HMRC's phones. Good luck with that!
;-)