Let's suppose you're in a Partnership with one other individual and they leave during the tax year, what happens? As can be implied, the Partnership ceases to exist from that point onwards since there's less than two active partners at that given time.
Please note that if a partner leaves a Partnership where there are at least two active partners remaining, the Partnership does not cease and may remain.
How would you approach this scenario on your tax return(s)?
You'll report on the 'Partnership (SA800)' up until the date of cessation (the date in which the partner left the Partnership), the remaining time period would then be declared on your 'Self-employed (SA103)' page on your Personal Tax Return. So for instance, if your partner left the Partnership on 5th October of a given tax year, you'd declare the Partnership's income from the time period 6th April - 5th October, then any income thereafter until 6th April the following year would go on your SA103 page.
What would you do if the second partner is refusing to give you the relevant details you need to complete the SA800?
You'll need to submit any income from the time period post-cessation under your own 'Self-employed (SA103)' page as you can't submit the SA800 without the second partner's UTR.
You'll also need to contact HMRC to cancel the Partnership since you're unable to do so via the SA800 form submission.