I think I have been accidently entering wrong tax & National Insurance

calcotti said:

Does this help you
How to report your earnings from self-employment - GOV.UK (www.gov.uk)
If you think you have misreported information in previous assessment periods you could inform UC and prepare a new schedule with correct information. They can then recalculate your entitlement.

Everything is on a cash accounting basis.

I'm not clear who is automatically deducting 20% from your earnings.

calcotti said:

Got it. The 20% is deducted and passed to HMRC as an advance payment of any income tax and NI liability you may have.

Seems to me you could enter the information in one of two ways
1) show your gross earnings and income and then record the 20% deduction as a expense, or
2) simply show the net amount you receive as income.

1) seems more logical to me. You can always ask UC to confirm whether there is a preference.

However you cannot simply write in that you have paid tax and/or NI if you haven’t actually made a payment. It's cash accounting. If you later have to pay some extra tax you would enter that as an expenses in the assessment period during which you actually make the payment.

From the link I posted earlier

Income Tax
This is the amount of Income Tax you have actually paid to HMRC on your self-employed earnings during an assessment period.
You don’t need to estimate how much you owe for the month. If you haven’t actually paid any Income Tax in the assessment period, you should report £0.

National Insurance contributions
These are either Class 2 or Class 4 contributions for National Insurance that you have actually paid on your self-employment during an assessment period.
You don’t need to estimate how much you owe for the month. If you haven’t actually paid any National Insurance in the assessment period, you should report £0.

calcotti said:

I thought that DWP sent self employed claimants details of how to report their earnings.

if you want to regularise it you will need to go back through your records and work out what you should have reported each assessment. Inform UC that you believe you have been reporting incorrectly because you misunderstood what was required. Give them the correct information. They can then recalculate your entitlement for each period and compare it with what you were actually paid which will allow them to work out any overpayment or underpayment.

if you do nothing DWP may well pick up discrepancies when they data match with HMRC in the future.

You should make a copy of all the important messages in your journal to date, particularly all of the existing payment statements. If UC recalculate your entitlement the existing statements will be overwritten but you may want to be able to refer back to them - hence the need to make copies.