From RAIL magazine No197 (March 31 - April 1993) Picture by Phil Precious.
Found Issue 198.. Apparently the 91 had failed the day before so the Parcels Class 90 that should have worked the above parcels had worked the previous days 14.00 Edinburgh - London KX bound train. In order to get the 91 back to Bounds Green it worked this parcels train as far as Peterborough in leu of the 90.
Sound like a fair swap! That sort of cooperation would not happen today!!