Hi,
Accumulations dividends are entered in the 'other dividends' box 5 on page TR3 of SA100. UK authorised unit trusts, open-ended investment companies and investment trusts, these are paid without tax deducted and include the full amount of these distributions in box 2 on page TR3 of SA100.
Have a look at the SA150 guidance notes for the self assessment tax return.
How to fill in your tax return
Thank you.