Hello, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
Firstly check HMRC's figures for accuracy. If they are incorrect, a not unusual occurrence, then write back pointing out the errors. If they are correct then bite the bullet and pay up promptly. Amounts owning, if less than 3K, may be adjusted and recovered through your tax code.
I do hope that you have found my reply of assistance.
The address given on HMRC's letter confirming the amount owing. These days you will almost certainly find it is a South Coast address, a BX9 post code. The second part of the post code directs any correspondence to the correct office.