The first photo shows my black 3.5 inch chain pulley. I had to add the second green pulley to stop the chain hitting the bottom of the seat and the return chain derailleur mount.
The second photo is without the black pulley but just the small green one.
Both combinations work on the bench and I'm thinking of ditching the black pulley and just leaving the green one. Just wondered if anyone could think of a problem with this?

I've noticed using the black pulley can stop me getting to the bigger cogs on my rear freewheel (especially if I use a bigger v pulley than 3.5 inches as the return chain hits it. If I just rely on the green pulley then I remove the problem and the two chains can't clash.

third picture shows the rear of my trike just to show that I'm making progress