Basically the mk IIIs in an HST are multiple unit vehicles, and those on the sleeper (or the Chilterns) are loco-hauled stock. To use an example I T S will be familiar with, this makes them as incompatible as 80 class trailers and NIR mk II stock from the Dublins pre-1995. The ludicrous reclassification of class 253/254 to 'class 43' makes it confusing as it suggests that HSTs are in some way just loco-hauled trains rather than semi-fixed formation DEMUs.