A bit of a misleading response there, on my part, with the benefit of hindsight. Yes, a four-car DMU set has been covering an EMU duty this week, but, conversely, a Class 319 has been doing the one Airport to Preston via Chat Moss duty, which is diagrammed for DMU. So, swings and roundabouts.
Out of Northern's currently available 27 319s, 24 are scheduled for use on Mondays to Fridays (this doesn't include the one currently doing Airport to Preston). Three seems a likely number to be required for scheduled maintenance (the equivalent totals for 323s are 15 required for Monday to Friday use, 2 required for scheduled maintenance).
So the deployment of Northern's 27 319s seems to sum up as follows: 24 required for service Monday to Friday, 3 required for scheduled maintenance, nil general spare. Whether nil general spare should be considered sufficient leaves scope for argument!
Actually, by virtue of the logic that a DMU can work an EMU duty but not vice-versa (the current one exception is a consequence of unusual circumstances) I would be able to see the point of not keeping any EMU spare, and covering with DMU if needed. But that, of course, assumes that spare DMUs are available..