At this time of year a lot of the time its been one on exam and one on tyre turning so effectively 5 from 5, this also means that the 5 serviceable sets run out of exam miles quicker as they're never spare.
There's also the Leominster incident which although a reformed rake is back, has impacted things quite a lot, using resources to get HD02 back out but also losing half of HD08, which should have been out around now but will now be around 10 months yet, this set is cover for the heavy overhaul programme which has also subsequently been knocked back by a similar amount of time awaiting HD08. There's also been animal strikes which knocked HD05 out for a long period, tree strikes which damaged a few sets and other incidents like HD03s severe wheel damage which, whilst they're back (HD03 should be back mid week) they've all impacted maintenance and exam schedules by losing sets for relatively long periods.
Loads of work has gone into keeping availability good and for quite a few periods, diagram coverage has been in excess of 90%. Obviously as a caveat there's been a bit of a wobble the last few weeks prior to this week due in the most part to some of the above issues. Compared to other small fleets, such as AT or 230s they generally hold up pretty well. The big difference is its far more obvious when a 2 car 197 rolls in vice a Mk4 than a 2 car 150 vice an Active Travel unit so is far more noticeable and affects far more people.
Comparing them to the 197/1 fleet which has 22 diagrams for a 26 strong fleet but generally only manage cover around 15-17 diagrams a day, a fleet thats causing shortforms its certainly not restricted to Mk4s.