Thanks Brian
Have tried a quick adaptation of your solution and it’s already close to what I am looking at.

Another quick one, when I render preview, as you can see from the attached screenshot the BG blocks of colour are very ‘sharp’ and not as blended as the desired result. Can any bluring be applied to acheive this result ?
Also, in regards to shadows, is there are particular method to have just the floor of tunnel in shadow (again like the example) instead of displaying in the roof and the walls?
Thanks again for help.