Bit of a guess here, but I would suspect that one of the two portions that would have been combined in March Down Yard to run to Mountsorrel was running late and the opportunity was taken to add the on time emptys to a SCN train returning to Toton.
The hoppers could then have been deposited in Mountsorrel sdgs en route for loading more or less to time (or maybe even early) with only a proportion arriving late back at Mountsorrel.
These hoppers worked fairly intensive circuits and late loading would delay the next outbound and if an opportunity existed to get them back to Mountsorrel earlier it would have been taken.
The 20s back in 1988 would be more likely to have been the portion locos forward of March to Kennett or Barham with the main loco (a 56) going to Trowse, so maybe the 56 failed resulting in the late running ?
As regards the being tempted by an N gauge version, oh yes, cheque book/credit cards at the ready, bring em on.
Regards
Scott