Some mk3s survived with Chiltern and Arriva after that I think.
The Mk3s were mostly 3b FOs, and were bought by Arriva. A number were converted to standard class and formed most of the ATW loco-hauled fleet (they have only recently been withdrawn). There were also some Mk3a RFMs, which mostly also went to Arriva - these were mostly not used, and several have now been scrapped. One of them, 10202, was bought from Arriva by the 125 Group who are converting it to act as a support coach.
The single Mk3 sleeper mentioned above went to Scotland for spares for their sleeper fleet, I think.
Not sure who bought the Mk2s, or the Mk1 kitchen car.
The Chiltern Mk3s are all Mk3a (one is ex-HST). The core of the fleet was ex-WSMR, and was expanded using vehicles which Arriva already owned, plus several from the ex-DRS Mk3 charter set, which Arriva also bought. Those used were Mk3a FOs, and were converted into TSOs, with power doors, etc, fitted at the same time. I think one or two of the ex-DRS set might also have been used by ATW, and also converted from FO to TSO.