Platform tickets only seem to be valid for 15 minutes now, at least the ones I have used are. The main use of them is to meet someone presumably. It doesn't really matter anyway - if I'm snapping a train I tend to use my local bridges, don't really like platform shots.
I dont know about the particular platform ticket in question lasting 15 mins, however I have in excess of 100 platform tickets from around the country (all bought within the last year on APTIS machines, and they have all lasted 60 minutes, and with the exception of Leeds, all cost 10p- Leeds' are 20p). I have noticed barriered stations are often reluctant to sell the platform tickets, and often I have had to explain that I collected them, and on more than one occassion in London, I have had to show my travelcard - which sort of defeats the object of "normal people" buying Platform tickets!?
I have also bought a couple on the new TIS', including "Fujitsu Star", and "Cubic FasTIS" and both clearly state on them "Valid for 60 minutes blah blah, NOT VALID IN TRAINS".
But really, from experience, t/o staff do seem to think your either a fare dodger, or "strange" for buying platform tickets!!! (They are (or at least were a couple of years back) in the fares manual btw @ 10p for 60mins).