Any question answered...

If you do freelance work and invoice for that work within a tax year, but the company doesn't pay the invoice to you until the following tax year, do you declare/pay tax for it in the year that you did the work and invoiced or the following year when you received the payment?

If your accounts are on the accruals basis, the former. If your accounts are on the cash basis the latter. I can't remember what the rules are for self-assessment and the cash/accruals accounting basis - I had to switch to accruals many years ago so haven't bothered remembering the qualifying factors.

Edit: Just checked - £150k limit to cash accounting for most sole trader businesses, £300k hard limit if you've already been using the cash accounting basis historically.