(25 Jan 2016, 7:08 pm)passenger_10 wrote But what about used vehicles that enter the fleet or transfer between depots? A used bus coming into the fleet may be due its annual test in the days or weeks after it arrives, but the bus it's replacing may be on the schedule to be tested several months into the future. Presumably each bus's MOT only lasts for 12 months and you couldn't run a bus without a valid MOT just to fit the depot's engineering schedule. 

I would imagine a second hand bus arriving would just have to continue on its old schedule so that its MOT doesn't lapse, or pass its MOT before it enters service, depending on circumstances at the depot with engineering / service requirements?

Clearly in the reverse situation where the bus being replaced is shortly due its test and the bus coming in still has several months to run on its MOT this isn't an issue, other than the potential cost of preparing and testing a bus soon after its already been tested.

Vehicle movements have their MOT date taken into account when they are planned for transfer or withdrawal. In the case of Stockton, the outgoing SB120 buses were in most cases withdrawn in the order their MOT was due, although a couple of them might have been due to major work being required which weren't cost effective to repair when there were other vehicles that could temporarily remain in place.
It is known that new vehicles can have their first MOT months ahead of their expiry, but when you get large batches launched (e.g. the fourteen Streetlites at Redcar), these need to be planned this way to ensure all are completed within their first year.
When a transferred vehicle is brought into a depot, it will be seen which is the nearest space to it's test expiry to be allocated. If the available space was many months early, they would simply  move other vehicles forward a week or two on the plan to create a gap for it to be sensibly placed in the MOT order. Depots like Stockton have an advantage with this as the plan covers two vehicles a week, with gaps where only one is up for test. This means it is easier to fit vehicles in on quieter weeks and swap around when only one vehicle is up for test.
The only issue is when incoming vehicles (especially new ones) are delayed. This can lead to a vehicle being tested and only lasting a few weeks before withdrawal. The only way round this might see a temporary loan from another depot to cover rather than spending best part of a week preparing a bus for test.
I think this covers the points raised. Hope this makes sense!