What I thought was the assembly part you were looking for in my box of spares was a 4-barrel carb from my old '67 Firebird. Damn, I've been looking for that thing for 20 years! If you want it, you can have it.
Anyways, I did some WEB searching. I found Potterton Home Page, a house for sale w/a Potterton Diplomat, and a few Heating sites (mostly in the UK but there's 1 in Texas too.) I even found the resume of a guy that use to be an engineer on some Potterton project, or something. Hopefully, between all this, you can get what you need.