Hi Peter,
Getting back to your class 20s, I always have trouble getting the body on and off, especially the cab end. I have improved it by filing all the edges of the chassis and cab body where they touch, it is a very tight fit.
Your 7mm stock is looking good. As you can imagine my O gauge is stored at present, garden railway shut for winter and no chance of getting to New Milton club layout for a run this winter with the lockdowns. So concentrating on my loft OO which is 1960s at present and it is finally getting some scenics after all these years.
Best Wishes
John