Well I'm coming to this discussion a bit late but here goes...
1415 should be used on the X12 when required and while I understand why Durham prefer a high capacity vehicle (ie Lowlander) they should be using the branded spare vehicle, ie 1415. Stockton don't have a branded spare so we try to put a vehicle on that closely matches the spec which will usually see one of 1491/2/4 used when required.

1512 is a branded spare for the 5s which when not required will be thrown on a town service (ie the 27) so that it is never far away when it is required... We usually know in advance if a Streetlite needs to come off so that another bus can be taken over to Middlesbrough and replace 1512 on the 27. If that happens to be a Cadet then another swap can be arranged with a Pulsar on another service to have a suitable vehicle by the end of the day for the X5 that it's regular board operates. I would have expected Durham to utilise their spare Sapphire vehicles in the same way, presumably a town service such as the 43/46/48 would be the best place to put them so that they are available at short notice. Obviously Durham should know best where to put it (I'm aware some of the 43s and 46/48s do schools and therefore require double decks).

As for the Northumberland MAX routes, I think they're trying to fit the capacity to the routes but we simply have a lack of double decks obviously. Maybe time to look at boosting some frequencies? The X15 has gone all DD for the summer but this has obviously meant that another route has had to lose out, the X14 in this case.

I don't really understand the "quick links" branding on one Scania and X14 branding on the other - they're wanting to convert the X4/X5 to MAX next but again there's a lack of suitable double deck stock for that which is why 1402/3 are branded currently I would assume. Will they upgrade some 12-13 year old single decks for that? I'm not convinced but then again the Scanias are 10 years old now. A logical swap in my eyes would be Stockton's 4652/4/8 to swap with 1401-3 which would mean all of the Scanias would be MAX spec (apart from the 07 plates, not sure what will happen to those now!...) and the Commanders would have a good home on the 21A. I'm biased, of course, as I would love a go of a Commander having ridden the Yorkshire ones many times.

As for the elderly double decks, the Lowlanders are down to be replaced this year by more cascaded London double deckers, I just hope we get something a bit less fragile than the B7TL's we got last winter. Some of the 04 plate DAF Geminis would be a superb replacement for the Lowlanders, and I expect the next things to go, though probably into 2016, would be the batch of DLAs we got in early 2013. By that time we would have got 3 years out of them which is a decent extended life for cascaded London stock. Single decks tend to last longer but there again those X-GGO Darts are getting binned at the moment!

PS: Sorry for the essay