OMG, this problem has followed this airplane round the world and it seems over 10 years. It appears it had two intermittent in-flight faulty up-lock switches. (That's the ones on the up-lock hooks). I replaced both these switches and no more problem. Unfortunately I replaced only one at a time which cured the problem for a little while, all the time the second switch was failing more often as the doors would not close until the handle was recycled (sometimes twice).
It seems to me that vibration in flight was causing the switches to fault, they wouldn't on the ground or even on the test bench. It's a cheap fix, probably worth doing before anything else. On the positive side, it did mean I had the power pack overhauled, emptied of 50 years of sediment and the 72 O rings replaced which is probably going to save me some grief in the future.
If you want to see what this airplane is up to now go to
www.outbackovernight.com