Hi Donna Findlay,
There are two ways to maintain self employment records.
You can use standard accounting practice, where you record income and expenses by the date you invoice clients or were billed.
Business records if you're self-employed
Or by using the cash basis, where you declare money when it comes in or goes out of your business.
The cash basis, has limitations and does not suit all businesses.
Certain types of business are not permitted to use it.
You will need to check whether you declared on your tax return which method you used and whether the profit declared, is based on the appropriate method of recording.
You are able to amend tax returns for up to two years after the due date of the tax return in question and in writing, requesting overpayment relief for the two tax years prior to them.
SACM12150 - Overpayment relief: Form of claims