W9135E was a regular on the Portsmouth - Bristol / Cardiff run from as soon as that lost the Inter City dmu sets around 1968. It remains the only Gresley vehicle I have knowingly travelled in and was, at the time, freshly outshopped in blue-grey with a red moquette interior. It was replaced eventually with a Mk1 RMB before the buffet service was withdrawn from that route.
The Manchester Piccadilly - Harwich Parkeston Quay boat train was an out and back working for many years requiring only a single set of coaching stock. This included a Gresley buffet for some years which I recall was E9131E.
The others which remained in traffic (of which I only knowingly saw E9124E) certainly did turn up at Kings Cross and later in the Highwayman which terminated at Finsbury Park but I don't recall seeing them in the Cambride sets. The Thompson buffets did work to Cambridge however and a photo of one (1706 I believe) was published in The Railway Magazine in 1970 or 71. I no longer have access to copies of that age to confirm more specific details.
Not all of them reversed in Norwich (Thorpe) station. The avoiding line was still open at the time with some summer dated trains booked to use it. Double-headed 25's were the norm. The last time I made such a trip was from Yarmouth to Doncaster via Reedham (not stopping!), Thorpe curve, Ely avoiding line (the U-turn), March and Spalding via the Axeholme Joint! You haven't been able to do that for a few years.