I've just started my PhD and gained my first installment of my stipend, which I understand is tax free. What I'm unclear on is whether or not I need to inform the tax office that I am no longer working and on an annual tax exempt stipend or if I only need to contact them should I take up an additional part time job. I've tried looking online but the information seems to be unclear and conflicting. Any advice would be much appreciated!

I've contacted HMRC any time I've taken on bits of work, as the tax codes can get quite complicated when you're doing various bits of casual work, but you don't need to tell them specifically that you're getting a student stipend. Your funding body should process this without any need for a tax code, and your previous employer should notify HMRC that you no longer work there.

Thank you so much! That clears up all the confusion, much appreciated!

Just to add that should you take additional work will studying if they don't use the normal tax code something 10 which means you don't pay tax below 10k per tax year, just keep your pay slips and apply for a refund at the end of the tax year. Your stipend has nothing to do with your taxable earnings. If you only ever work casual jobs (a few hours a week) even if you do that for the whole year you'll unlikely earn enough to be taxed so remember to claim it back.