Am I right in thinking that Heaton is used because it has the spare space after a lot of its allocation’s routes have been converted to Tyne and Wear Metro operation over time?
Space is the primary reason for Heaton being used to carry out many of the heavy overhauls on Neville Hill and Newton Heath allocated units. I'm not familiar with the setup at Newton Heath or Allerton, but certainly Neville Hill has more facilities than Heaton has.
Heaton's allocation actually grew when Metro opened, as prior to the opening of Metro the DMU fleet was allocated to Gosforth. Though many of the DMUs were transferred directly from Gosforth to other regions, some had to stay in the North East to work the remaining unelectrified lines in the region, and those units transferred to Heaton. Prior to the opening of Metro and the resultant transfer of DMUs from Gosforth, Heaton was primarily a HST depot.
When the East Coast Main Line was electrified Heaton lost it's HST allocation - Heaton's HSTs being transferred to the Midland Main Line and their replacements on the ECML being Bounds Green allocated 91+Mark 4 sets. Though 91+Mark 4 sets stabled at Heaton, not being allocated there, there were never as many of them at Heaton at any given time as there were HSTs prior. Around a decade later Heaton ceased to be used to stable any XC stock, the loco hauled stock and HSTs previously stabled at Heaton being replaced by Voyagers stabled at Tyne Yard.
The only Northern units Heaton regularly has to stable is it's own allocation and more than half of it will be outstabled elsewhere at any given time, whereas Northern's other depots have to stable some stock from each other's allocations in addition to whatever percentage of that depot's own allocation is usually there at any given time - both Allerton and Neville Hill stable Newton Heath allocated units on a daily basis, and Newton Heath stables Neville Hill allocated units.
Those factors combined, likely along with others that I've forgotten (and probably some I don't know of too) contribute to Heaton having more space available than most UK depots.
Im 99.9% certain it had already gone up to Heaton before that....over a week ago at least and I seem to remember noting it on RTT allocated to the 5A60 departure from York which is listed most nights on RTT...I was equally surprised to see the allocation myself and wondered why a 155 was heading to the NorthEast
Apologies...that appears to not be the case.. back at the end of December it was 155343 which was showing as allocated but it looks like it never materialised
155s share Hull - York work with Heaton allocated 158s. What likely happened will have been that the 155 ended up being swapped onto a service that should have used one of Heaton's 158s, and was automatically filled in for the remainder of the diagram. The allocation will have been subsequently manually checked, it spotted that the 155 was on a diagram that would (if not altered) send the unit up to Heaton, and arrangements quickly made to ensure that the correct units ended up in the correct place. It would be very unusual for anything other than a 158 to appear on the 5A60 working you quote, and if something other than a 158 were to turn up for it I'd expect the driver would likely query the allocation with control before taking the unit, even if the unit was a class that they signed.
It's not too uncommon for such allocations to appear on the likes of RTT, but very rare that such an allocation will actually be the unit that turns up.
