Following on from these two previous posts My Dell Inspiron 8500 Screen Is Dead :( and My Dell Inspiron 8500 Screen Is Dead Again, I can now report that thanks to the feedback from my Dell Inspiron 8500 screen is working again! The same issue that happened to him had happened to me. The switch that monitors whether the screen is up or down had come away from its mounting point and once I removed the plastic strip at the top of the keyboard and remounted the switch the screen worked again! Many thanks MarsKy :)
So I've got a couple of days work copying files from my replacement laptop back to my old laptop and re-configuring my old laptop again. I'm keeping my fingers crossed that this is a permanent solution this time.