Just spent half an hour trying to figure out why it wouldn't compile before I realized I still had the board selected as UNO ... (facepalm)
Anyway moving on. Just to confirm did I also need to modify the board.txt file as per what raalst tried first or was the change to the platforms.txt all that was needed? I've changed both and it's working like a dream but I won't leave board.txt modified if the change wasn't needed.
I've been finding and comparing different PID and FFT libraries for the DUE and to have them so neatly pre-packaged and (according to the docs) cortex optimized is way beyond what I expected to get, so big thanks raalst & cmaglie. I'm going to be trying to FFT a piezo sensor input to identify vibration modes in machinery as a fault finding/early fault detection system. If I get it right I'll make a post for it.
Cheers
Andrew