Hi Alex. from what you've described, it sounds like you already have the pics etc that I have somewhere. However, there is a publication covering British Gun turrets that could help, and also the book 'The Turret Fighters'. Sorry, can't remeber the author or publisher, but, coincidentally, I saw this was still in my local library only a week ago! I think there are some details of the turret in this book. Another source might be the R.A.F. Museum, Hendon, who have on display the only Defiant extant. Their research department might be able to help. There is also a collection of turrets, and assocated info, at the Yorkshire Air Museum, Elvington, near York. Although prossibly not exactly the same, the turret on the MkIII Halifax was a Boulton Paul 4 gun turret, and the feed/ ejector mechanism etc will be very similar. I believe there is such a turret at Elvington, plus the Halifax of course, so a call/visit might provide what you are looking for.
Hope this helps a bit.
Terry.