Thanks Giz you were spot on.
I had undone the 4 screws at the end of the bogies which I now see secured the black body metal to the chassis. I didn't see the other four!
Lesson 1: work in a better light. Lesson 2: study the diagrams better.
Next problem I have is that one of the black wires is detached from the head code light. I don't think I did that as I never got that far so don't know if it was designed like that ie with just 3 attached and one not connected, or should it have all 4 like the other end? It will be a pain to re-solder though.