"I have received an APN, what should I do?"
APNs are a statutory demand of a payment of tax upfront (regardless of whether the dispute is still ongoing). It cannot be appealed to an independent court.
An APN gives you only 90 days to make a payment - it can have a drastic impact on your finances. However with the right guidance on how to deal with HMRC their impact can be mitigated.
APNs are statutory demands - doing nothing is not an option.
You should consult with a tax professional to help you manage the impact of an APN.