Before you file — who must file, and the one rule that's different for a deceased person's return
SA100 isn't something a personal representative starts unprompted, and the deceased's copy of it doesn't follow the usual online/paper choice. Confirm both before doing anything else.
A return only exists once HMRC asks for one
Section 8 of the Taxes Management Act 1970 creates the duty to file only once HMRC gives notice requiring a return.
A personal representative shouldn't assume a final SA100 is automatically needed — HMRC will tell them, and send the paper form, usually once notified of the death.
Paper only — HMRC will not accept an online return
GOV.UK is explicit that HMRC will not accept an electronic Self Assessment return from a personal representative for a deceased taxpayer.
Whatever software or online account you might normally use for your own return, the deceased's return goes in on paper.
The deadline is in HMRC's letter, not the standard dates
Printed on the form itself: the standard deadlines are 31 October for a paper return, 31 January for an online return, each after the tax year ends (or 3 months after the date of HMRC's notice if that's later) — with a £100 penalty for late filing, £10-a-day daily penalties once more than 3 months late, and interest and a late-payment penalty on top.
Neither standard date automatically applies here. 'The return must reach HMRC by the date given in the letter you received with the form' — set case-by-case, not fixed.
Who actually signs it
The personal representative — executor or administrator — signs personally, entering the capacity they're signing in at box 23 of the last page.
HMRC's own printed example for that box is 'executor, receiver'.
SA100 isn't something a personal representative starts unprompted, and the deceased's copy of it doesn't follow the usual online/paper choice. Confirm both before doing anything else.