Wasn’t it the case that eventually a decision was made to have all trains running at maximum length, and this was basically achieved by not having a full range of spare coaches. Hence whenever a coach failed it was usually just removed for repair, and then returned. So there’s never really been the same sort of day to day re-shuffling of stock between coaching sets, as with say HST Sets...
I think that's basically correct.
The loss of numerous vehicles in the Hatfield and Heck accidents saw a number of vehicle conversions to give 30 full sets, leaving just a spare standard class 'end' vehicle and a spare DVT. Hence if individual vehicles require maintenance over and above the norm this almost always sees sets operating short-formed as there simply isn't the flexibility to shuffle vehicles between sets.
In a nutshell, and in no particular order, the 'Mallard' refurbishment saw the following alterations:
21 open standard vehicles (124xx & 125xx) were converted to open first vehicles (112xx, found next to buffet)
7 open first vehicles retained their original numbers (112xx, found next to buffet)
all remaining open first vehicles were re-numbered to either 113xx (open first disabled, centre of 3 first class vehicles) or 114xx (open first next to DVT with smoking area)
2 buffet vehicles (10314 and 10316) were re-numbered to 11998 and 11999 (open first, found next to buffet like 112xx)
remaining buffet vehicles retained their original numbers (103xx) and had standard class seating fitted to replace first class
1 open standard vehicle (12531) was converted to open standard disabled (12331)
all open standard disabled vehicles (123xx, found next to buffet) retained their original numbers
all remaining open standard vehicles (124xx and 125xx), open standard end vehicles (122xx) and DVT's retained their original numbers, and in the vast majority of cases were extensively shuffled between sets compared to pre-Mallard refurbishment
During the Mallard refurbishments with numerous vehicles stopped, I vividly remember seeing sets in service without a buffet vehicle at all and in one case a buffet vehicle next to the DVT.
Interestingly the ECML HST sets, aside from the addition of an extra standard class vehicle and one or two shuffles due to minor accident damages, remain primarily in their pre-Mallard refurbishment formations to this day.
Pre-Mallard refurbishment something tells me it was 8(?) 'pullman' mk4 sets which had 3 full first class vehicles instead of the usual 2 - I seem to think these were typically used on up London services in the morning peaks, covered Leeds or Newcastle return trips in the day, then formed down London departures in the evening peak.
Happy to be corrected on any of the above - particularly the 'pullman' info.
Hope this helps.