1 day late:
A fixed penalty of £100. This applies even if you have no tax to pay or have paid the tax you owe.

3 months late:
£10 for each following day – up to a 90 day maximum of £900. This is as well as the fixed penalty above.

6 months late:
£300 or 5% of the tax due, whichever is the higher. This is as well as the penalties above.

12 months late:
£300 or 5% of the tax due, whichever is the higher.
In serious cases you may be asked to pay up to 100% of the tax due instead.
These are as well as the penalties above.