Self assessment penalties explained.
If you file your self assessment tax return after 31st January you will receive a £100 fine. If it is 3 months late you are then fined £10 per day for up to 90 days. If it is 6 months late you are fined another £300 or 5% of any outstanding tax (whatever the higher) and again if 12 months late, another £300 or 5%.
If you are late paying your tax, in addition to being charged interest at 3% you are fined 5% of any outstanding liability on the 1st March, another 5% of any outstanding liability on the 1st August and another 5% if still unpaid by the following 1st February.