The 'Army turning-up' at Heathrow happened several times during the 70s ... the first few times would appear to have been immediately after some kind of incident elsewhere in the world, but several other times it happened as a 'reminder' that the security forces were out there, and available at short/no-notice.
My OM used to work at Heathrow and had many tales of run-ins with the Army. It seems he'd show his ID badge to the first Army bloke he encountered, and continue on his way. When he was next challenged he'd just ignore them and carry-on as if they weren't there! They'd get very jumpy, so he'd simply drive-off in his van (he was 'airside' and could go pretty much anywhere he wanted!), leaving them fuming.
In fairness, he was 'ex-Services' himself, so he knew what he could and could not do, and just how far he could push them.