I found the reference - it comes from 'Milk Churns to Merry-Go-Round' by R.T. Munns. He talks, breifly, about the difficulties of the operating up freights through Peterbourgh and goes onto say 'During the second world war this hazard was eased somewhat by a siding between Spital Junction and Peterbourgh North being upgraded to a running line and duly signalled to enable a freight to creep forward to Peterbourgh North and dash through the platform line as soon as a margin between passenger trains permitted it.' Sorry but no mention of if they continued to use it after the war.
Rob