But what is the impact for each 197 that has to cover a Mk4?
As I say, from the outside, each cover is a double unit that now can't be doubled because 1 of the 2 197s needs to go cover. So 3 covers on the Marches means 3 other Marches (or a 197 service somewhere else) are now losing a 197. In any day how many 197s are used to double up on the Marches? My back of the envelope maths estimates, 5. The specific units change, of course, but it is how many are needed for doubles between Swansea and Manchester. Of the 5, 3 pairs are split and essentially, you could only have 2 pairs left. Hence on a day where there are 3 Mk4 diagrams short, even if 197 performance meant the other two were also singles, the Mk4 would have had the bigger impact on shortforms.
It would just be helpful to understand the impact that each Mk4 failure has so an objective judgement can be made on why we see planned 197 services (e.g. 1W12) shortformed.