A knackered starter motor will often spin with no load but not develop enough torque to turn the engine over.

Something like this? https://www.johncraddockltd.co.uk/gts004-single-half-tow-step-black-finish.html

This is the one I have on mine. I know it is not strictly a Series 3 step but I think it looks ok and it acts as a useful rear bumper.

You need to have made at least 50 posts on the forums before you can advertise your vehicle for sale. @blue beasty may be along to have a word soon.