Reading depot: white carriage identification 2018-11-30

Today (30 November) my wife travelled through Reading. She reports that there was a white carriage in the depot but was unable to give much additional detail. After much questioning, it seems that it might have the appearance of a SWT-liveried intermediate vehicle (i.e. no red/orange swoosh). It apparently carried the number 1 (first class).

It doesn't sound quite like any barrier/translator vehicle I know. Can anyone identify it, please? Thanks!

MK III 10229 (believe it’s an ex Greater Anglia coach)

Came down from Crewe yesterday (hauled by ROG 37884)

At some point it will come down to Long Rock until the last buffet of the sleeper has finished

Thanks both. I'll have to find an excuse to make the same journey very soon to try and see it!