Great post Phil , Mixed emotions for me as I worked at Royal Mail when Mayfield was still in use,usually on night shift so you can imagine the added atmosphere it created,The area below the platform was known as "The Rathole" and in peak times was used to process mail order returns,It also stored various items of Royal Mail memorabilia such as the old Penfold post boxes, I recall seeing a couple of green Victorian ones there,The mail-bags would arrive from London and be off loaded on the platform,The mail bags would then be hung upside down on to a chain which travelled from Mayfield Station via the overhead conveyor bridge you mentioned,however it did not go to Piccadilly Station it went to the Parcelforce building on St Andrews St, Im sure this was the longest chain in Europe,Once inside the Parcelforce building the mail bags were cut open & the contents went down various chutes dependant on the labels tied around the neck,Nice to see the old tiled staircase again,that was particularly spooky at night and The Star & Garter would have an open (back) door policy for the Posties during those long nights.