I self assess through Government Gateway. Although the deadline to submit and pay is 31 January following the end of the tax year, I like to get mine done early, asap after 5 April. My tax affairs are straightforward.
It’s much easier nowadays with online pension portals providing P60 data and banks and building societies providing online certificates for taxable interest. Any other income, SP, dividends, some freelance earnings which are not taxed at source, I keep a record of throughout the year.
HMRC know what SP someone has as DWP tell them but it's worth noting that you are assessed not on what you receive in the tax year but what is due. That's because SP is paid in arrear.
The SP is increased from the first Monday falling after 6 April but because it's paid in arrear, depending on payment date, you could receive one four-weekly payment at the old rate and 12 four-weekly payments at the new rate. The amount you need to return is 13 x the new rate as that is what is due.
So long as you have all the data to hand when you log into Gateway, the step-by-step self-assessment process is easy. I usually file mine late at night or in the early hours when I know I won't be disturbed.
2024/25 took about 30 minutes including double and triple checking everything. The HMRC statement agreeing liability arrived a week later.
The HMRC investigation time limit is 4 years if an innocent error is suspected. Where mistakes in tax returns are deemed careless or negligent, the window extends to 6 years. Suspicion of deliberate tax evasion warrants an investigation period of 20 years.