HI Chris,
I'm in the process of doing one of these myself, except mine is sharing its chassis with my old Triang L1 albeit with modern wheels off the Hornby schools class.
I got the idea from K Chadwicks book, and have replaced the chimney (LNER flowerpot from 247 developments),dome and reversing rod(Hornby B17) and smoke box door(home-made).
I'm not happy with the smoke box not being circular like the real D16 and I'm contemplating having ago at rectifing this ASAP, as well as getting a replacement smokebox door from Southeastern finecast, as they use them on their N7.
Regarding the B12 tender, I've managed to pick up a couple off ebay in the last month at about £5 each.
I'll post a picture of it, when I've worked out how to do it.
Cheers, Ian