I've never personally experienced it on an HST or a Mk3, and travelled extensively on HSTs on the cross country routes and on Mk3s from Euston in the early 90s. The first you normally knew the PA was in use was "Good evening ladies and gentlemen, this is your senior conductor speaking....."
I also don't recall a BR refurbishment in the '90s I seem to recall it was the late 80's that introduced the pink theme into first and the orange into standard.
Indeed this video is from the '80s and shows the theme... (admittedly in Mk2s).
http://www.youtube.com/watch?v=vv640aRwFS8
Looks like the TOCS introduced their own chimes in as part of their refurbishments (from the mid 90s)
EDIT. A video from Intercity days on an HST... have a listen from 2:40 to the stewards announcement. That's how they used to be.
https://www.youtube.com/watch?v=tDMoDq8x1vk
But I agree the bing bong was on the 225 sets from new.