From memory when the HSTs first came out you could work out all of the individual trailer numbers from the set number.
The power cars W43042 and W43043 were formed in set 253 021. (Where S is the set number, the last digits of the power cars were 2S and 2S+1.)
The last digits of the seconds were 3S, 3S+1 and 3S+2. That gives W42063, W42064 and W42065.
First class trailers would have been 2S and 2S+1, so were W41042 and W41043.
Catering vehicles were harder, but probably included W40021.
I will have to look up my records for more detailed information.
Geoff Endacott