Does anyone know what the issue is with the SQ773 at LHR is. It must of been there for well over 2 weeks now.
I think you are mistaken. All of the daily SQ 777s that have visited LHR in the last couple of months have left the same day.
For the last 3 weeks or so it's been the SQ306 (STA 0720) that has gone out on the evening SQ305, so it's normally towed to a remote stand in between.
Prior to that, when it comes in as SQ308 (STA 1510) it normally stays on stand until departure. Maybe that's what has led to the confusion.