Welcome Carl -- You've been left an ideal starter kit for this hobby, & you#ve found this list so you have started off right...
Not sure where you are locared, but we are right in the middle of rally season. Best start is to get to a local rally, & chat with folk there. The D (or A,B,J or whatever similar Listers) are very rugged, & easy to work on & get spares. Only likely problems are usually caused by poor maintenance (no oil, winter frost damage caused by leaving water in engine during big freezes). The only other common problems are typical of any old engine that's not been used for many years -- magneto needs cleaning & possibly remagnetising; carb jets all bunged up with crud; piston stuck; oil seals dried out etc
Work Google to death (text & pix). A search on Bamford corn mill gave plenty of hits this morning, so that would get you started.
Get a copy of Stationary engine magazine. May be in your local newsagent if you are in a rural area, but no chance in Chelsea. Try their web site (Google again). They have a good booklet on the D's ( & also others on different Listers). Gives history & tips. They have done many articles over the years on mills, & I suspect their farm equipment publication has also covered them many times.
Rebuilding mill is pretty straightforward. First photograph everything in detail. Then peel back slowly, cleaning as you go. Keep photographing, & watch for broken castings, chipped teeth etc -- in case the missing pieces are in the next pile of dirt you are about to clean off & throw out. If bearings completely shot, several ways to rebuild -- shout here if necessary for pointers. If you have gone back to bare metal, paint with primer before the rust grabs hold. Don't wait months between strip & rebuild, or those photographs will be even more important, & the cat will have run off with key small parts in between .....
Check the mill & the Lister out. You'll find engine numbers & machine models on them. Record them, as you'll need them to track down fellow owners of similar kit.
Two really good sources for research data are the Museum of Rural Life at Reading University. Online, worth checking out the website of the Musuem of Power in West Wales
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The latter has a strong literature section, & good links etc.
Enjoy. I'd gather info, find owners, & watch others until the fall, then get stuck in over the winter ready for next spring's season. Whatever you do, there are folk willing to help if you get stuck.
Last tip -- there are some engine owners who are patronizing towards Listers & the D in particular. Ignore their prejudice & stupidity -- the D was probably the most reliable & numerous engine to be found on farms everywhere. A true farmers best friend, & much to be encouraged!
'best,
Colin