This morning I noticed I had a notification in Play Store so I clicked on it and it's a 99p Movie Rental deal. I'd love to redeem this offer but the trouble is I'm not sure how I find out which titles are eligible for it. Any help?
Welcome to Android Central! Those deals are usually for any movie rental. Select a movie and tap the rental button -- if the deal is valid, you'll see the reduced price there before you complete the transaction.
Just search for any movie in the Play Store (look along the bottom of the main screen for the Movies category), and if it's available for rent, you'll see the rental price. Not all movies are available for rent -- many recent movies that just became available to purchase aren't for rent yet.