So we’ve swapped the 430W for a spare 430 and, based on testing with a hand-held 250m metres way, that seems to have fixed it. We will see if any more complaints are made, and in the meantime the 430W goes off for testing.
@alioth was absolutely right about needing to be several hundred metres away with the hand-held; closer you get breakthrough even with a known-good radio, and sitting in the cockpit is really no good at all.
