Hi Adrian,
A P45 would usually cover your earnings for the tax year up to the point you leave that employment. Are you saying yours only covers from Jan ’23 to your leaving date, so you’re missing details from April ’22 to Jan ’23?
If so, you could try speaking to your previous employer to question this and see if they can supply more detail.
You could also try setting up a personal HMRC login (if you don’t already have one) and see if you have access to your PAYE record there. Alternatively, you could speak to HMRC directly to ask for a ‘statement of earnings’.
If you’re filling out our Self Assessment questionnaire, please supply the Gross. We’ll also ask you to attach your documents if possible.
All the best,
Katie