In 1974/5 I was fortunate enough to "cab" the prototype HS. The set was in the long siding at Chippenham early one afternoon, I was passing, so asked for a look in the cab.
I spent quite some time on a conducted tour and having my questions answered, before being told to hop off, unless I wanted to cycle back from Swindon !
My even more train mad little brother was very jealous when I told him about my adventure ...