You get 10 months to complete a rather simple form where you declare any income. I am really struggling to see where the issue is even with OAPs. If a pensioner has a single pension then the form will be a doddle. If they have multiple/complex income streams then maybe a family member or friend...
Why didn't you simply return the form stating your only income was PAYE? This would have sorted everything and then put the onus on the HMRC to prove otherwise. Self assessment is not a difficult form to complete. All you have to do is enter any income streams you have so if you were only on...