The plain buttons are no indication, I'm afraid; it was very common for British dinner jackets of that era to have simple black horn buttons with a raised rim. I must admit, from my monitor, it looks as though the lapels are a different fabric from the rest of the jacket, and I would have said a grosgrain with a fairly pronounced horizontal rib - is that the case? If so it is definitely a dinner jacket. You should certainly try for the trousers, but in my experience dinner jackets are often, for some reason, separated from their trousers. I hope you find them. That said, black barathea of that era is easier to find a match for than many fabrics. As to Austin Reed, many British men's clothing retailers started off offering at least a bespoke or made-to-measure option, not excluding Burton's. Indeed, Austin Reed still offers one. That's a handsome jacket. I think that you said the dinner jacket was made for the same person? If so it is interesting to note that they could afford to move their custom from Austin Reed in the 1940s to Savile Row in the 1960s! I haven't tracked down much information about Sandon & Co; they feature, I'm sure, in Richard Walker's Savile Row Story, but they aren't in the index and the only reference I have found is to them offering (along with Poole's to make cheap uniforms for troops in the Boer War, thus taking them back to at least 1900). However, I believe they were a well-respected firm, albeit long defunct or merged by 1988. I used to own a 1927 tailcoat made by them, which was very good. With the coat and the possible dinner jacket, you have some nice pieces - well done!