If your GST turnover for FY 2019-2020 is £678,000 but Form 26AS shows £1,200,000 credited under Section 194C, you should declare your actual net turnover as per your books of accounts in your Income Tax return. Form 26AS might show a higher figure because it doesn't account for purchase returns.
30 August 2020
You need to show the actual net turnover as per books of accounts as Form 26AS may show higher turnover as these are not accounted for purchase returns.