Thanks Ilder.
We've done a lot of teamwork to pln saves too and this is the first time it's happened. Existing sections seem to be fine so it's just the new ones that seem to have this problem.
We've tried re-saving pln files from the same model a few more times to see if the problem appears with these attempts (never mind what they say about madness being doing the same thing again and again, expecting different results). Curiously, the problem replicates itself in all of these attempts except in the 4th and final pln we saved.
Now after saving the model out 4 times, we have one that works. Weird..