Purnell has signed a deal until the end of the 2018/19 season.
On his contract extension, Purnell told the club`s official website: “The club is in a good place both on and off the pitch. It’s not often you play for a manager and management that allow you to express yourself, even as a goalkeeper so it was a no brainer for me to sign.
"Playing for my hometown club is always a big thing for me and I’m delighted to sign an extension.”
Assistant coach Rob Boyd added: “It’s great that Mark McGregor has managed to secure the extension of Luke’s contract. This gives us stability to build upon. Luke continues to improve, and we’re sure the supporters all appreciate what an important part of the squad he is”.