COSSACK...The guys have a point, sometimes departing behind a B757...or...following one on approach can be an interesting experience . For us the cutoff is the defined 136,000kgs making an exception for the B757,generally, as one makes their way out for departure at LHR and the subject a/c is departing ahead, I would endevour to advise ATC of the 2 mins or even 3 if using B16 or B79,sometimes I've felt the lash of ATC's razor tongue but that a/c does churn up the air a lot ...Please fogive us our...