Sheen marks a transition point where all flights switch to their Instrument Landing System (ILS) on the final approach.
No, it doesn't.
During the day, any flight that is not established on the ILS by 8nm from the threshold (roughly overhead Putney in the case of the southern runway) is automatically classed as a "late join" and reported as such in the airport statistics.
For 2012, late joins accounted for about 1% of arrivals. So even if all the offenders were flights coming in over Sheen, that would still mean that more than 97% were already established on the ILS well before they got to you.