I've just come across your Essex post and must say it is good to read what an outsider of my area has to say!
Of note from your post, you might not know that 65672 (and the rest of that batch of 12) used to be dedicated to the 100 Chelmsford to Lakeside service many years ago. Back when used to operate every 20 minutes as opposed to every 15 minutes now.
On the history of 65031 (and the other 4 in its batch) it began life plying its trade on the Sandon Park & Ride in Chelmsford (hence the leather seating) before being replaced by Eclipse 2's following the opening of the Chelmer Valley Park & Ride, with the OmniCity's dropping into the general Chelmsford single decker pool. Incidentally I challenge you to find a pool of buses that can cover such a varied area from 1 depot with Chelmsford single deckers reaching; Brentwood, Colchester, Southend-on-Sea, Stansted Airport and Southminster...pretty much most of Essex!
Ensignbus are a master of success, considering they originally had no interest in a local network in Thurrock originally...they managed to pick up emergency tenders (from the Town & Country demise) and slowly managed to run Arriva out of town who now maintain a local depot here for TfL work only!
The Arriva KT Sapphire vehicles have resulted in the previous Wright Pulsars now being transferred to my "home" depot of Southend. Wayyy back in 2008 were stored at Southend making staff here very jealous (driving E reg double deckers) before they moved over the water to Northfleet.
Finally, I am curious as to what you find interesting about Basildon depot because to me it just seems a bit of a mish mash of vehicles :)