You were indeed confusing it with Mode 4, old bean!
State of the art aluminium pursuit ship indeed
I'm afraid that Walter's theory of little green men, USMC or USN Seals from Macrihanish scuttling about the Mull planting IFF beacons is clouding the issue somewhat.
But perhaps there'll be something released under the FoI which may give support to Brian's unrelenting efforts for justice.
Personally, my suspicions are that any 'Not Positively Determined' finding would have resulted in the next of kin of the deceased suing the MoD for very considerable sums. Whereas the grotesquely offensive 'Gross Negligence' finding would have laid all liability with the deceased aircrew.
Keep at it, Brian!