Two sets of 8/321's ran in service today, 404 and 406 which went back to Hornsey this morning after covering for 387's due to a lack of crew knowledge, the afternoon diagrams were formed of 8/387 which for the first time are booked to berth at Cambridge Carriage Sidings for tomorrow morning.
407 and 408 have worked all day vice 365's and will finish at Peterborough Nene tonight plus 420 and one other 321 are also on the Nene. Both pairs are likely to be working vice 387's tomorrow and likely to be diagrammed back to Hornsey in the morning.
At the moment, they may last until the end of this week but will only be used if crew knowledge dictates them replacing 387's or to cover to for 365's but their use will be very limited.
12/321 did indeed depart Hornsey EMUD bound for Clacton depot but evidently did not get further than Ilford.
As for the use of 387's to Kings Lynn, at the moment they will only run as far as north as Cambridge and their use on diagrams other than booked trains will be extremely limited due to crewing issues. You are more likely to get a 317 to Kings Lynn than a 387 for the foreseeable future and that will be extremely unlikely.