On the subject of 'stinking' HSTs, another early olefactory issue was the smell of the brakes, the pungent-acrid reek of whose resin was very efficiently distributed throughout the interior thanks to the air-conditioning. Worst when applied quickly at high speed - a not uncommon occurrence in the early days when very high speeds were commonplace. (I travelled between Reading and London daily for much of the late seventies and early eighties, usually returning on the 23.00 ex-Paddington, a particularly fast run where the hasty application of the brakes was far from rare: I can still recall the resultant reek.)
The introduction of the 'InterCity 125' service certainly increased passenger numbers on the Western Main Line, and they were really comfortable (especially while they still had the predominantly four-seats-to-a-table layout before all the extra airline-type seating was shoe-horned in to increase seating capacity.)
Although I'm far from the Thames Valley now, I'm still very much aware of HSTs, since the Dundee - Aberdeen line, whilst not visible from the house, is well within earshot, and four 125 services in each direction make their presence strongly felt as they comply with the "Whistle" boards as they rush towards the occupation crossing down to the beach.
As a sadder footnote, could I mention one other modification that was added to HSTs after introduction? The central door locking system. Even in the earliest days, some journeys out of Reading were standing -room only, and many standees chose to stay in the end vestibules with their end-hinged doors. There were unfortunate cases of people falling to their deaths from these doors whilst the trains were at speed, whether intentionally or unintentionally, and it was suggested that the doors might be opening of their own accord whilst people leant against them - from personal experience I can certainly say that the big diagonally sloping grab-handles on the doors were quite comfortable to perch on if there were no seats available. There were petitions made about the doors' safety, and eventually the locking systems were installed. **