Hi Steven - I think I may have narrowed it down to the departure airport (NZQN) - I just retried a Simbrief plan to NZQN from YBBN and the TO Perf worked for that one.
I reconfigured the flightplan for NZQN to YBBN and the TO Perf doesn't seem to like it, whichever option I select. Screenshot is attached. Thanks for your help.
Regards, David