If you are found to owe HMRC a tax debt, then as with many debts, quick action will help you avoid some of the more severe consequences.
In the first instance, contacting HMRC and discussing your position is always a good option. This can help you avoid any penalty charges that may be applied to your debt.
Penalty charges are applied when you are 30 days late, then again at 6 and 12 months late.
This charge is applied like interest, at 5% of the original debt you owe.