Gordon's Engine. How do I seperate the chassis from the engine?

[Portugal Joao]

I have an old Gordon which runs well in reverse.I need to remove the chassis from the engine to get at the wheels on the tender since the middle ones are loose. How can I do this? The whole thing appears very stiff and I cannot seperate the metal from the plastic.

 

[96RAF]

Download service sheet 269 which may help. I think on this model the middle wheels are wobbly.