I have recently (within last 3 weeks) accepted an offer for a PhD studentship, however, now I'm not sure whether I even want to do a PhD this year, I feel like I need a year out from study. If I withdraw this year from that studentship, how will this affect any applications next year?

Contact the university and ask about their policy of deferral.

hey claire,
first of all congrats on your offer. Most of the Universities allow their students to register till july (of the the subsequent year) with the studentship. (Moreover most univ have registrattion at janurary and april also). In that case you get 8 months off without deferring

If you have no doubts about the project itself then defer, but DO think carefully about whether it's the project you want to do next year too. Withdrawal will put off that institution from taking your application next year, but other institutions won't know about it so you can still apply to them. If you have the slightest doubt about PhD study this year - don't do it - take the year out. I did and I'm glad of it. Now I have a good project I'm 100% sure I want to do.

P.S: If you're going to withdraw/defer, do it a.s.a.p. so they can get someone else for the project if you don't want it.