Well... after 5 years (!) of doing other things, I have finally come back to this. The fact that Bachmann did an RTR City in the meantime has changed things a little (I have 6 lol).
So I have come back to this with a twist. It is now being built into a Flower class not a City. Deeper frames, moved safety valves, and accepted compromise with the boiler from the Airfix City kit being a little too big for a Flower. So it will be a 1920s vintage Flower - superheated, top feed, green/black/not lined/GREAT WESTERN tender livery. Not sure on which Flower yet - I have a stack of photos but I'm still deciding. Possibly 4166 Polyanthus.
Current progress:
Challenges so far:
1. The Airfix smokebox is too short... section added
2. The Airfix cab sides are not long enough, so I have added a small filler, and by the time the Branchlines brass cabsides are overlaid it looks ok
3. Because of 2., the roof is now slightly too short... looks OK though, I think I am just going to put a plasticard rim on the front edge so it is not flush with the cab front
4. I've not reverse-punched all of the rivets... having not done this before I missed the instruction to do that and soldered in the steps with the reverse side, at which point I thought "**** it" I'm not taking them out and doing that again
5. The chassis.... well.. the instructions are less than brilliant. The chassis sides have a figure 8 shape hole into which axles bushes can go either high or low. Instructions say to use the top ones, so I soldered the bushes in, squared up the chassis, and then tried to match it and wheels up to the footplate... Ummmm No. With the chassis mounted, the wheels foul splashers, the ride height is wrong, the inside frames are all but dragging on the rail heads. So they came out... try again. I asked Branchlines and they repeated the instructions from the kit - use the top holes.
Yeah... nah. Checked the GWR diagrams thank you Russell GWR Locomotives book, checked a heap of photos, and tried again. Using the bottom holes achieves a perfect ride height, the chassis fits properly, the wheels sit inside splashers, and the inside frames are at a far more photo-proven height from the rail-top, not dragging on the ground.
The tender is complete also - Zimo decoder fitted, with sound, speaker, etc., all wired in and running to a four pin plug for the loco.
It's been a long time coming, but there is a glimmer of promise that this may get finished this year!! :-D