Hunsdon had a Category 2 BABS Mk.1F (aka Airborne Interception BA), Cat 2 meant it was being installed. Marker beacons were not needed with BABS as the radar operator could read off the distance-to-go.
It didn't have Standard Beam Approach - it had VHF BA, Cat 1A (Installed and operational), and for what it's worth the crystal frequency was 6450.5860.
This means that it almost certainly would have offset markers. All SBA and VHFBA used vertical aerials for the Main Beacon - which gets you onto the runway centreline. All SBA aircraft had a corresponding vertical aerial for the R1124 Main Beam receiver.
SBA marker beacons had a horizontal aerial, which meant that all aircraft had to have a horizontal aerial for the Marker Receiver, R1125.
Fighter aircraft (limited space) used their R/T set with its standard vertical aerial as the BA receiver. They had no need (or room!) for a separate marker receiver / aerial, and consequently had to rely on 'stray' pick-up from this vertical aerial. If the marker (horizontal aerial) was in its usual place - on the centre-line - there would be no pick-up as the aircraft flew over. Hence the marker transmitter was positioned at the side of the approach and squirted its signal across the path so that the vertical aerial would detect something.
Think we may have it. (The BATF was No.1532, which then moved to Wittering - hence it must have been a Fighter Command VHFBA unit rather than the overwhelmingly common SBA training flights.
Finally I've found my SBA training manual which states the Inner Marker should be 150 yards from the threshold. Hence the 500 ft estimated looks about right. I'll go and have a beer!