The unit you saw was almost certainly 325005.
It’s at the country end of Platform 2, further up than normal, as there are 3 uncoupled units in a platform designed for 12 carriages (3x 4-car units)
Interesting chat with a guy doing the wraparounds on the 325s. He says they’ve been given 4 weeks to complete the work, and that removal of the Royal Mail branding/identity was a condition of the sale.
He genuinely doesn’t know who the buyer is, but expects to get some branding decals to put on as the last part of the process.
Alas, I’m on an extended holiday now, so won’t be able to offer any updates until late April… by which time the 325s may have been moved out of PRDC
(I was mistaken about the locks. The square panels are where the distinctive blue lights that indicated if the roller shutter was open were sited).
Interesting to note this article
https://www.ft.com/content/9a40dc0c-d96a-4a47-b7ea-ae1623627276 about high speed freight, in the Financial Times. It mentions Varamis Rail starting a new service between Birmingham International and London Liverpool Street in May…