Heaton used to maintain HST power cars and Mark 3 coaches, but this was removed in the early 1990s and moved to Craigentinny
I've seen a few GNER sets stabled at Heaton, but nothing more than that, and most recently it has been Class 91 and Mark 4 coaches
If there is maintenance of HST and Mark 3 coaches still underway at Heaton, then the diagrams don't allow for it
My main point is about "heavy maintenance"
I understand that Heaton can probably handle overnight maintenance on BT10 bogies, as is required at least once a week, as well as checking and inspecting, as is required by an A exam
However, rostering maintenance could be quite difficult as it will have to be performed wherever the spare set is or overnight
For example, lets say the buffet car suffers a hot axle box on the first trip southbound
If the spare set is available at Ferme Park, then this releases the faulty set for repair at Bounds Green
However, if the spare set is at Heaton then this may result in the train having to be terminated also affecting the northbound, whilst it returns to Heaton
Furthermore, from what I can work out, there is no spare buffet car, which is why I used this as an example!
Given there will be nine HST power cars with six in use each day, two on spare, and one available, that allows plenty of flexibility on a day to day basis, the problem is where to keep the spares!