These are probably questions for an ATCO such as HD:
Is there one ATCO assigned to manage an entire LHR holding stack? Or is there a team of ATCOs who work together to process aircraft through the stack before handing off to LHR tower?
Since aircraft are under radar vector at this stage of flight, does the controller tell the pilot to turn 90 degrees left/right or is the pilot responsible for knowing the procedure for that particular stack and when turns should be made?
Finally, if I may (!), what happens if one aircraft in the middle of the stack wishes, for whatever reason, to hold his altitude? Does this mean all aircraft at a higher altitude must also hold their altitude? Or is it still possible to descend aircraft at a higher altitude through that level without breaching the legal separation limits?