From the lhr.... website:
AC866 goes down to 3369 feet displayed on base leg before coming back up to 4232 feet shortly before intercepting.
Taken from the succeeding 747: 3925 feet displayed equals 4000 ft QNH, so the lowest of AC866 was around 3450 ft QNH.
While Mode-S altitude resolution is 25 feet (Mode-C 100 feet), where do these altitude data come from?