They have 8 sets. It's not unreasonable to expect more than 3 of them to be in use at any one time. Granted there's the ongoing overhauls which are reducing availability, but it shouldn't be reducing availability to as low as 37.5% which is what it currently stands at. 5 sets per day working would be 62.5% utilisation, definitely not an unreasonable expectation. If 37.5% availability is seriously the best they can get out of the fleet then some serious questions need to be asked about what is going wrong. I'm not accepting the move to Neville Hill as an explanation for it - whilst it no doubt doesn't help, a depot move shouldn't reduce availability that drastically, especially not one that took place 8 months ago now. The move of 158s to Heaton in 2019 didn't go anywhere near as badly from a stock availability perspective, nor did the move of XC's HSTs to Laira (although admittedly that was helped by Laira already maintaining other HST sets).
The main issue with the lack of them running north of York is with disruption. There have been multiple cases where recently 5 car sets (or sometimes even no set at all) have turned up for busy Newcastle services during disruption solely as a result of the inability/refusal to use the Mark 4 sets North of York. Newcastle based crews not signing them causes issues South of York too. Lets say (I know this might not be the most realisitic example but it should make the point) there's disruption, which leads to only one train and one crew being at Kings Cross ready to take a service North. That train is a Mark 4 set, and the crew is Newcastle based. Despite there being both a train and a crew ready, the service ends up going nowhere as the competency on the Mark 4s for the crew waiting has lapsed. Situations like that are fully preventable, and are of zero benefit to the customer.
Nobody (or at least not me anyway) is suggesting removing mark 4 sets from the Leeds route to run to Newcastle instead. Operationally it is more convenient to use them on the Leeds route, and where they can be used on the Leeds route without causing undue inconvenience for customers then by all means that should be where they are used, but operational convenience and saving money on crew refreshers and an extra thunderbird should not be taking precedent over getting paying customers (many of whom will have paid more than £100 for their ticket) to where they're going in a reasonable timeframe. Most passengers couldn't care less whether it was an Azuma, Mark 4 set or a brick on wheels taking them to their destination as long as it gets them there without unreasonable delay - passengers in the main will not consider "wrong type of train" as a vaild reason for a delay or cancellation.
