If the 'Harrier' was an AV-8 of the USMC, that would lead me to think that the accompanying mystery craft would have been of terrestrial origin and from the USA. Allegedly Macrihanish was used for other test-flights of the enigmatic Aurora spotted by that ex-observer corps chap on an oil-rig some time in the early-90's, so perhaps this was another such flight with an accompanying aircraft to muddy the radar returns should any ATC become overly interested.