The vehicle allocation for the X7 has been previously discussed on one of my photos on Flickr. When suggested that the X7 will use the plain yellow B10BLEs currently based at Percy Main, things escalated quite quickly.
Apparently, Peterlee uses MPDs and only MPDs because the drivers are paid less than at a normal depot. Transferring the B10BLEs over would mean that some of them would be driving normal sized vehicles and their salary would have to increase. This doesn't count in other depots because they have other vehicles too, but Peterlee just has the MPDs.

I don't know how reliable this information is, and I sadly can't give anyone the link to the photo in which these comments were made because it got quite aggressive on there too so I deleted all of the comments made which were off-topic to an MPD being on the 56!