1) no. Balfour Beatty have two Drain Trains, both consisting of two container flats with various modules on. Until the acquisition of the 20s, both drain trains worked within "Plasser TRAMM" machines, or sometimes between two "Plasser Dynamic Track Stabiliser" machines*.
The set currently between the 20s are BB's first drain train wagons dating back to 2004, numbers quoted above by ExRes.
The Bletchley train was the grey "Sussex Rail" drain train with wagons 95490 and 95491. They were acquired by Balfour Beatty in ~2019 and subsequently repainted blue & white.
2) *the DTS machines mentioned above were exported in December 2023 due to the acquisition of the 20s. They were sometimes used on BB's electrification trains, and hauling cranes & other items of plant about from depot to depot. It's not entirely unfeasible in the future that the 20s may also take up similar tasks when not in use on drain trains and is hinted as such in
BB's press release.