As an old lag who interviewed graduates (PhDs too) in large numbers, and still does a few (despite being retired) I would say:
1. Yes, mention the PPL. Briefly, and under "Interests".
2. Be ready to talk about it if asked, but don't bore the interviewer if he/she isn't interested.
3. Be ready for a snide remark (if you have that sort of interviewer) about the spendthrift aspect of it. Yes, if I had an overconfident candidate, I would turn snide. Part of the "tactic" to get inside their heads.
I used to review piles of applications/CVs, and decide which ones I wanted to interview. Those who had done nothing interesting were more likely to go on the "reject" pile. A PPL would probably get you an interview; once you're in the room, it becomes irrelevant except for what it reveals of your abilities and motivation.
Far more interesting, to a good interviewer, is "what leadership roles have you taken on?" Captain of sports team, organiser of UK Universities Ladies Everest Climbing Expedition (yes, I had one of those), or anything that shows you can lead and motivate people.
Your academic ability is a given - the Ph.D. shows that. What I would want to know is "what are you like as a person, and would I want you in my team?"
Happy to comment further off-list if you wish.