Sorry to step on HD's anticipated reply, but it was 'LH' in the late 60s/early70s. I remember seeing it on my first visit to EGLL on a Trent Bus Co day out from Loughborough. We had a guided tour around the perimeter road. It was amazing. I had never seen jet airliners before. The guide told us that the beacon, (or pundit), could be seen from over The English Channel. It was this bus trip which made me decide to become an ATCO.
A year or so later, (1970ish), my dad and I went to EGLL at night, with a friend who drove freight down there 4 nights a week. We stood on the grass outside the 'Hunting. hangar for an hour or so watching Argosies, Merchantmen and DC8 freighters landing on 28L. No security. I remember this beacon lighting up the whole sky with green flashes.
We had a smaller 40W model at EGNX, west of the old tower. It flashed 'CA', also in green, I think the RAF beacons were red.
Why were they called 'pundits' though?