A five coach train failed and the only thing available was a 3 coach 166 (you will probably find the only spare train was a 2 coach 165 which was swapped with a 166 which was already in service to free up the 3 coach 166 meaning some services were 2 instead of 3 and yours was 3 instead of 5 but I don't want to confuse the issue

).
During the peaks there is no spare stock just sat in sidings doing nothing, everything that can be out is out and the controllers/depot staff have got their fingers crossed that everything keeps running.
Exactly that!
Just to give you a little insight to our depot - as per the current diagrams, we have 16 units return to depot after the peak period, from approx 0930 to 1500:
5 of those units will be turned round, shunted into new formations and back in service by 1140.
The others will be a mix of pre-planned maintenance and units that have been outstabled the previous night(s) so require cleaning, fueling and any defects sorting.
All will be back out in traffic by 1700 for the evening peak.
Any units stopped from the night before, eg B exams, Component changes (Engines, Transmissions etc) very rarely sit spare for long after they are finished and prepped, Control will have them off us to replace any shortfall / failures out in traffic.