I get tax taken off through CIS. Do I still have to fill in a tax return?
Yes! CIS applies to the self-employed, who have to fill in a tax return every year. You may want to use an accountant to help with this process, or you can do it yourself online if you're happy to pull together the information required. If your income is below the higher rate threshold (around £40,000 or so) then filling in the tax return might even get you a tax refund. If your income is over that amount then you might owe money to HMRC – and you do have to pay this on time (not least because it saves hassle and it saves interest and penalties!).