Once a person dies all their bank accounts are frozen and then closed.
You need to ask the bank what happens next. As it was a business account I think the bank has to freeze the account while they find out if the business was owed money or had debts that need to be paid.
As asked, 'are you in the UK?' it makes a lot of difference . Law in the USA for example is a lot different from here in the UK. Your use of 'checks' rather than 'Cheques' makes me think you may be in the USA . I also don't understand the bit about ''but it is her tax I'd number'' Does not sound like a UK 'thing'.