Threatening Letters from HMRC are always a cause of worry for a business owner or Company director
Don't  ignore them , matters will only escalate, lead to demands for immediate payment of all arrears including a visit to your premises by HMRC or bailiff acting on their behalf

The first thing you must do is contact them explain the position of your position and agree a time to pay arrangement.

Explain to them that you wish to continue trading (assuming you do)  that the business is based on a viable proposition and that you need time to improve cash flow and restructure

HMRC are normally happy to collaborate with companies and businesses if they have confidence in the ability to pay arrears and will normally agree an extended payment time of up to 12 months in the first instance.

Keeping them informed is best practice