If you miss the January filing deadline for the online filing of your tax return, HMRC will charge you an instant £100 penalty.
In some circumstances, this can be reduced provided a valid reason exists for the delay in submission. TaxCalc features a reasonable excuse form, although HMRC find very few reasons acceptable these days.
Penalties can increase substantially until your return is filed and the tax is paid:
HMRC have the power to charge higher penalties for serious disregard of the filing deadline.