253's. They were originally (HST) classed as DMU's - but due to the amount of reforming that was made, they were reclassified as motor cars just carrying trailers as the class 43 HST. Not just class 43, as there was originally another class 43, it being the 'Warship'.
If 252001 is the prototype HST (which I think it is) then some vehicles from it ie MkIIIs are possibly still in use. I think one of the First HST Showcase vehicles is one from the prototype. The front end vehicles with their non-fullly streamlined noses are probably either dumped somewhere or were broken up. I doubt they ever ran in passenger service, they probably had a lot of equipment in for testing etc.
When the HSTs entered service they were classified Class 254 as well. AIUI the ECML ones were classified 254 and the GW ones were 253. The GW sets had 7 trailer vehicles and the ECML 8 hence the different classification.
I would guess they were renumbered in the very early 80s, or late 70s - I don't think they lasted very long as DMUs.