But with one unit away for refurb, and another at Doncaster, even a single unit being robbed can lead to short forms.
They simply don't have enough spare units to even rob parts from one, without causing short forms, or substitutions by inferior older trains
No, not quite . Its two away at refresh and 1 at Doncaster.
If we just look at the 166 fleet in isolation there are 21 units of which 16 are required for traffic. That gives you 5 units spare. Even with 3 away, that's two units left.
Not ideal as you still have to take into account routine exams & maintenance but not quite at the level you seem intent on repeating time & time!
What sort of fleet utilisation level would you like? 50% - one spare for every unit. 28% with only 6 in traffic and 15 sat on depot. I'm sure that would please the passengers as they pass on a packed out unit and see lots of units sat idle on depot "just in case".