Four pairs of unclued lights are all similarly linked.

                Across

5               Mark sample of class essays (6)

9               I’m beginning to like hot chap’s twinkling of an eye (10)

14            Beginning of oxidisation on iron container? (3)

17            One who chooses mixed ramen (5)

18            Of sound mind, regularly returns wearing gold ring (5)

20            Followers regret taking European money (7)

22            Half of Earth’s journey round Sun used for retaining heat … (7)

24            … is lacking energy in orbit (7)

25            Direct lines to the centre of Leningrad I installed (5)

26            Biting cold and dry outside (5)

28            Frank doffs cap to old money (7)

31            Shooting Night Hunter in...