Looked like it - was in Great Western Green.

Didn't know FGW had any 'fag packet' liveried HSTs left, I thought they
were all in 'barbie' livery now. If it is green, as you say, it might
have been some Mk2s and a Class 57 locomotive, which are still green.
FGW have been diverting some HSTs to Waterloo when there's been
engineering work between RDG and PAD in the past.